Chapter 11: Bleeding and Thrombotic Disorders

The coagulopathies are grouped into disorders of bleeding and thrombosis. The hemorrhagic diseases are further subdivided into the 2 major categories of coagulation factor disorders and platelet disorders. To understand the diseases with abnormal coagulation that follow, a brief introduction to normal hemostasis precedes the discussions of the diseases.

Normal hemostasis is the controlled activation of coagulation factors and platelets leading to clot formation, with subsequent clot lysis, in a process that stops hemorrhage without excess clotting (thrombosis). Effective hemostasis is a rapid and localized response to an interruption in vascular integrity (vessel wall injury), such that clots are formed only when and where they are needed.

Clot Formation

Clot formation involves platelet activation and the subsequent generation of fibrin via the coagulation cascade. The 2 processes are discussed separately in the sections that follow.

Platelet Plug Formation

Platelet plug formation is initiated in vivo by exposure of platelets to vascular subendothelium when a vessel is injured. The platelets adhere to the subendothelium, spread out along the surface, and release substances that promote the aggregation of other platelets at that site. The platelets also accelerate fibrin clot formation by providing a reactive surface for several steps in the coagulation cascade.

Adhesion of platelets to the subendothelial surface is facilitated by a plasma protein, von Willebrand factor (vWF), especially in vessels with high shear forces (eg, the fast blood flow in arteries has a higher shear force than slow blood flow in veins). vWF binds to a specific receptor on the platelet surface. Deficiency of vWF results in poor adherence of platelets to subendothelium. The severity of bleeding in von Willebrand disease (vWD) varies widely among patients. Another related platelet adhesion defect occurs in patients whose platelets lack the receptor for vWF. This bleeding disorder, known as Bernard–Soulier disease, results from an inability of platelets to bind vWF.

Platelet activation occurs from interaction of platelet agonists, most of which are soluble, with specific receptors on the platelet membrane. Physiologically important agonists include adenosine diphosphate (ADP), thrombin, epinephrine, collagen, and thromboxane A₂, which is derived from arachidonic acid. A sequence of membrane and cytoplasmic events is initiated by the agonist–receptor interaction, involving an increase in cytoplasmic calcium ion concentration and a platelet shape change from a disc to a spiny sphere. The change in cytoplasmic calcium concentration leads to contractile events in the platelet, causing alpha and delta granules (also known as dense bodies) to fuse with the platelet plasma membrane and release their granule contents into the extracellular space. Successful granule release requires the formation of thromboxane A₂ from endogenous arachidonate via the enzyme cyclooxygenase. This enzyme is inhibited irreversibly by aspirin and inhibited reversibly by a number of other anti-inflammatory agents. Treatment with aspirin can cause a platelet secretion defect (reduced release of granule contents) that is often clinically significant in patients with underlying coagulopathies. Alpha granules contain vWF, fibrinogen, Factor V, 2 platelet-specific proteins—platelet factor 4 (PF4) and beta–thromboglobulin—as well as a number of other proteins. Delta granules contain serotonin, adenosine triphosphate (ATP), ADP, pyrophosphate, polyphosphate, and calcium. The release of some of these substances, in particular ADP, activates unstimulated platelets nearby. Absence or deficiency of alpha or delta granules occurs as a feature of several congenital and acquired platelet function disorders, collectively known as storage pool disorders. Individuals whose platelets possess appropriate numbers of intact granules that cannot be released on appropriate stimulation have a platelet release defect, and on that basis also may have a bleeding tendency. Aspirin is a common cause of a platelet release defect, because it impairs thromboxane production.

Release of platelet granule contents is followed by platelet aggregation, that is, the binding of platelets to one another to form the platelet plug. Normal aggregation requires fibrinogen binding to platelets via the fibrinogen receptor, which is the glycoprotein IIb/IIIa complex (GP IIb/IIIa), on the platelet surface. Congenital absence of GP IIb/IIIa results in a bleeding diathesis known as Glanzmann thrombasthenia (GT).

The platelet surface serves as a site for certain coagulation pathway enzyme reactions (see below). For example, the platelet membrane can bind the Factor Xa/Factor Va complex that activates prothrombin to thrombin. Thus, platelet activation and fibrin formation via the coagulation cascade are interactive biological processes.

Fibrin Clot Formation

The coagulation factor pathway is an enzymatic cascade with sequential conversion of proenzymes (zymogens) to fully activated enzymes, which then convert other zymogens to their activated forms (Figure 11–1). The final steps directly preceding fibrin formation can be activated through both the intrinsic and extrinsic pathways; hence, this part of the pathway is called the common pathway. Numerous positive and negative feedback mechanisms exist in the coagulation pathways so that the cascades do not proceed in an uncontrolled fashion. The pathways are now known to be extremely complex, with multiple interactions between factors in the intrinsic, extrinsic, and common pathways. The following description is a version of coagulation factor interactions that highlights the fundamental reactions.

The coagulation cascade is activated by the appearance of tissue factor (also historically known as Factor III), which is not normally exposed. Tissue factor is presented when a blood vessel is injured. It binds with Factor VII and small amounts of circulating active Factor VII (Factor VIIa), resulting in a complex of Factor VII or VIIa and tissue factor. The Factor VII in the complex can be autoconverted to Factor VIIa, resulting in a greater number of Factor VIIa–tissue factor complexes. This complex activates Factor IX to Factor IXa in the intrinsic pathway, with some activation of Factor X to Factor Xa in the common pathway. Factor IXa, with support from Factor VIII as a cofactor (or Factor VIIIa, which is a much more effective cofactor), activates Factor X to Factor Xa. Factor Xa with the assistance of Factor V or, more effectively Factor Va, converts prothrombin (Factor II) to thrombin (Factor IIa). At this point, the coagulation cascade is markedly amplified because thrombin activates Factor VIII to Factor VIIIa, a more effective cofactor, and activates Factor V to its more effective Factor Va form. In addition, thrombin activates Factor XI to Factor XIa, which, like Factor VIIa and tissue factor, activates Factor IX to Factor IXa. Thrombin catalyzes the conversion of fibrinogen to fibrin, which is then cross-linked by Factor XIII to create a stabilized form of fibrin (clot). Factor XIII is also activated to Factor XIIIa by thrombin.

Factor XII, prekallikrein (PK), and high-molecular-weight kininogen (HMWK, shown as HK in Figure 11–1) are not required for the generation of fibrin in vivo because even when they are completely absent, there is no increased risk for bleeding. Nevertheless, the intrinsic pathway is activated when Factor XII contacts collagen (exposed by vessel injury) or polyphosphate (released from activated platelets).

This cascade, as it is currently understood and described above, explains 2 long-standing clinical observations. First, it explains the significant bleeding tendency associated with Factors VIII and IX deficiencies because these factors are important in the early stages of cascade amplification. Factor VIIa and tissue factor activate Factor IX to Factor IXa, and Factor IXa requires Factor VIII to convert Factor X to Factor Xa. Second, this scheme provides an explanation for the clinical observation that deficiencies of Factor XII, PK, and HMWK are not associated with an increased risk for bleeding because Factor VIIa/TF activates Factor IX, and thrombin activates Factor XI, bypassing the need for Factor XII.

Two of the coagulation cascade reactions occur on the platelet surface. The first of these is the activation of Factor X to Factor Xa, which is produced by platelet-bound Factor IXa and platelet-bound Factor VIIIa. In the second, platelet-bound Factor Xa and platelet-bound Factor Va convert prothrombin (Factor II) to thrombin (Factor IIa) in a subsequent step in the coagulation sequence. As noted below, single factor deficiency states, most of which are congenital, exist for all of the factors, but multiple factor deficiencies, which are usually not congenital, are much more commonly encountered. Inhibitors, as antibodies directed against a specific coagulation factor, can arise to any of the individual factors to create deficiency states. With some exceptions, most factor inhibitors are rare.

There are 2 major inhibitory pathways that determine the rate at which the cascade is amplified. One of these is the protein C–protein S anticoagulant pathway (Figure 11–2). As shown in the figure, excess thrombin, which is generated through the activation of the coagulation cascade, provides the signal to shut off the coagulation cascade by binding to a protein on the endothelial surface known as thrombomodulin. The thrombin/thrombomodulin complex converts protein C into its activated form. The activated protein C then binds free protein S as a cofactor. Protein S may be bound to C4b-binding protein and to a limited number of other proteins, in which case protein S becomes inactive. Once free (unbound) protein S binds to activated protein C, the activated protein C/protein S complex then proteolytically degrades Factors Va and VIIIa, reducing the flux through the coagulation cascade by removing these 2 activated cofactors. A mutation in the Factor V molecule, known as the Factor V Leiden mutation, makes Factor V resistant to proteolytic degradation by the activated protein C/protein S complex. This condition is known as activated protein C resistance. This permits the prothrombotic action of Factor Va to persist and contribute to a hypercoagulable state.

An additional mechanism for control of the coagulation cascade involves the inhibitory action of antithrombin (formerly known as antithrombin III) (Figure 11–3). Antithrombin has a limited anticoagulant effect on its own, but in the presence of heparin or selected other negatively charged heparin-like molecules, antithrombin adopts a new conformation that increases its inhibitory activity 1000-fold, permitting it to inhibit most of the activated coagulation factors in complexes where both antithrombin and heparin bind to the activated coagulation factor. Inhibition of Factor Xa, however, does not require the direct binding of the activated coagulation factor by heparin. Factor Xa can be neutralized when it is bound only to antithrombin, after antithrombin has been activated by heparin or related molecule. The antithrombotic action of short chains of heparin (low-molecular-weight heparin) is directed primarily against Factor Xa because short heparin chains inhibit predominantly only Factor Xa (longer chains are required for Factor IIa inhibition). Fondaparinux, a synthetic heparin-related pentasaccharide, inhibits Factor Xa exclusively.

It should be noted that all of the coagulation factors in the coagulation cascade are synthesized in the liver, and that the activity of Factors II, VII, IX, and X is vitamin K-dependent. Tissue factor is constitutively expressed on some cell types, and in other cell types, such as endothelial cells, it is not normally expressed. There is little (if any) tissue factor in the circulating plasma under normal conditions. It is not exclusively synthesized in the liver. Tissue factor in the subendothelium is exposed to blood when blood vessels are injured, triggering the extrinsic pathway of the coagulation cascade. The half-lives of the coagulation factors in the blood vary markedly, with Factor VII showing the shortest half-life of approximately 5 hours and Factor XIII having the longest half-life of more than 120 hours. There is also a wide range of plasma concentrations for the coagulation factors. Factor VII is in the lowest concentration of the circulating coagulation factors at 100 to 500 ng/mL. The highest concentration is found for fibrinogen at 200 to 400 mg/dL.

Fibrinolysis

Fibrinolysis is the controlled dissolution of the formed clot. It occurs when the injured vessel begins to heal, and is initiated to a limited extent when clot formation begins. In this way, fibrinolysis serves as a regulatory mechanism to limit excess clot formation.

The principal enzyme involved in fibrinolysis is plasmin, which exists in a zymogen form known as plasminogen (Figure 11–4). Plasminogen is converted into plasmin by tissue plasminogen activator (tPA). Plasmin degrades the fibrin clot. A recombinant form of tPA is used as a pharmacologic agent to produce thrombolysis (breakdown of a thrombus), in patients with myocardial infarction or stroke, and clots elsewhere in the body. Clinically effective derivatives of tPA from genetic manipulation are also used for thrombolysis. tPA is released by endothelial cells, and its secretion into the plasma is increased by thrombin. Plasminogen activator inhibitors are secreted by platelets and endothelial cells, particularly when they are activated. Plasminogen activator inhibitors stabilize a newly formed clot by blocking the action of tPA, thereby preventing premature dissolution of the clot. Plasmin degrades fibrin polymers, and, to a limited degree, fibrinogen as well, by specific and sequential proteolytic cleavages, generating fibrin degradation products (FDP). These FDP (fragments X, Y, D, D-dimer, and E) may be detected in the plasma of patients experiencing fibrinolysis. Deficiency of plasminogen may predispose to thrombosis, and deficiency of plasminogen activator inhibitor may increase the risk for bleeding. Antiplasmin inhibits plasmin, but only when it is circulating and not when it is clot-bound. This prevents the circulation of a proteolytically active form of plasmin, while permitting clot lysis to proceed by plasmin within the clot. Deficiency of antiplasmin may result in a hemorrhagic tendency.

Figure 11–5 provides a classification of coagulation disorders. The major division in the classification is between disorders associated with bleeding and disorders associated with thrombosis. There are 2 major subdivisions of bleeding disorders—those associated with coagulation factor and fibrinolytic pathway factor deficiencies, and those associated with an abnormal platelet count or impaired platelet function. Isolated factor deficiencies are usually congenital, although occasionally an isolated acquired factor deficiency develops. An example of an acquired isolated coagulation factor deficiency is the Factor X deficiency associated with amyloidosis. The deficiency of antiplasmin is listed in this section because its absence permits increased plasmin activity and overactive clot dissolution, resulting in a bleeding tendency. Another major category of coagulation factor abnormalities is multiple coagulation factor deficiencies. There are several commonly encountered situations associated with multiple factor deficiencies. These include vitamin K deficiency or warfarin intake (which results in a reduced amount of functional Factors II, VII, IX, and X as well as protein C and protein S); disseminated intravascular coagulation (DIC), which results in the consumption of multiple coagulation factors; and liver disease that results in decreased synthesis of coagulation factors. Several activated coagulation factors are inhibited by heparin. Heparin administration results in inactivation of most of the activated coagulation factors.

The group of disorders associated with platelets is divided first into quantitative platelet disorders and qualitative platelet disorders. Quantitative platelet disorders include thrombocytopenia and thrombocytosis. Thrombocytopenia can be produced as a result of increased platelet destruction, from a variety of immune or nonimmune causes, or decreased platelet production. Common causes of decreased platelet production include tumor infiltration of bone marrow from metastases or a hematologic malignancy, and drug-induced thrombocytopenia as occurs with chemotherapy. Thrombocytopenia can also occur as a result of increased sequestration of platelets in the spleen, usually in patients with splenomegaly. Thrombocytosis is much less common than thrombocytopenia. Thrombocytosis can be divided into reactive thrombocytosis, in which there is a transiently increased number of platelets from a stimulus to increase platelet production, or neoplastic thrombocytosis, as seen in myeloproliferative disease and, less commonly, myelodysplastic disorders.

Qualitative platelet disorders are characterized by abnormal platelet function in the presence of a normal platelet count. vWD is a disorder in which there is defective platelet function, but from a defect originating outside the platelet, since vWF is generated primarily in endothelial cells. vWF coats the surface of the activated platelet to allow it to adhere to the cut vessel surface and initiate platelet plug formation.

Other causes of defective platelet function result from abnormalities within the platelet. These disorders may be congenital or acquired. The congenital ones are extremely rare, and the acquired ones are very frequently encountered. Congenital platelet abnormalities associated with defective function include GT, Bernard–Soulier disease, and storage pool disease (SPD). The much more common acquired qualitative platelet disorders include drug-induced platelet dysfunction, such as produced by aspirin and clopidogrel (Plavix), and uremia-induced platelet dysfunction, which occurs in patients with impaired renal function.

Fibrinogen Deficiencies

Description

Fibrinogen is produced in the liver by hepatocytes. Abnormalities of fibrinogen production may be congenital or acquired and, in general, involve either decreased production of a normal molecule (afibrinogenemia and hypofibrinogenemia) or production of an abnormal molecule (dysfibrinogenemia) (see Table 11–1).

In congenital afibrinogenemia and hypofibrinogenemia, there is a reduced (hypofibrinogenemia) or absent (afibrinogenemia) production of a normal fibrinogen molecule. In general, the homozygous deficiency results in afibrinogenemia, and the heterozygous state results in hypofibrinogenemia. Both disorders are rare. Homozygotes suffer a mild-to-moderate spontaneous bleeding tendency. Manifestations include umbilical stump hemorrhage and bleeding from mucous membranes, among many other possible signs and symptoms related to blood loss. Severe bleeding may occur with trauma or surgery. Hypofibrinogenemic patients are usually asymptomatic, but may bleed significantly with surgery or trauma.

Congenital dysfibrinogenemia is a result of inheritance of a gene for an abnormal fibrinogen molecule, which is produced in normal or near-normal quantities. All the fibrinogen produced by a homozygote for dysfibrinogenemia is abnormal, and approximately half of the fibrinogen in a heterozygote is abnormal. Hundreds of abnormal fibrinogens have been described. The true incidence of dysfibrinogenemia is not known because many forms of the disorder are asymptomatic. Homozygotes may have a mild bleeding tendency, perhaps because the fibrinogen molecule is cleaved too slowly to form fibrin monomers or because abnormal fibrin monomers polymerize too slowly. The bleeding tendency is characterized by easy or spontaneous bruising, menorrhagia, and prolonged or severe bleeding with surgery or trauma. Heterozygotes are usually asymptomatic, but may show excessive bleeding with surgery or trauma. Several types of dysfibrinogenemia (approximately 10%-15% of cases) are associated with an increased risk of thrombosis rather than bleeding. A few types of congenital dysfibrinogenemia are associated with both bleeding and thrombosis.

Acquired hypofibrinogenemia is observed predominantly in patients with advanced liver disease, in patients with a consumptive coagulation disorder such as DIC, and in those treated with thrombolytic therapy.

Acquired dysfibrinogenemia represents the acquired production of an abnormal fibrinogen molecule in normal or near-normal quantities, most often in patients with acute or chronic liver disease, especially those with primary or metastatic hepatic malignancies. The patient may or may not be symptomatic, depending on 1) whether there is simultaneous production of normal fibrinogen in amounts sufficient to allow normal hemostasis and 2) whether the abnormal fibrinogen can polymerize like a normal fibrinogen molecule (see the section “Hemostatic Abnormalities in Liver Disease”).

Diagnosis

See Table 11–1 for the laboratory evaluation of the patient with a fibrinogen deficiency.

Prothrombin (Factor II) Deficiency

Description

Prothrombin (Factor II) is the precursor to thrombin (Factor IIa), which converts fibrinogen into fibrin in the common pathway of the coagulation cascade. Deficiency of prothrombin, either inherited or acquired, may result in a hemorrhagic diathesis. Inherited abnormalities of prothrombin are rare. As with fibrinogen, abnormalities occur in 2 major forms. The first is reduced or absent production of a normal prothrombin molecule. The second is production of normal amounts of an abnormal prothrombin molecule with decreased activity (dysfunctional form or dysprothrombinemia). Heterozygotes usually have approximately 50% of normal activity and may be asymptomatic or have a bleeding tendency. In 1 study, 83% of a small cohort of heterozygotes had bleeding, with Factor II levels ranging from 21% to 35%. Homozygotes usually have 1% to 25% of normal activity and have a mild-to-severe hemorrhagic tendency. Acquired hypoprothrombinemia occurs most often along with deficiencies of Factors VII, IX, and X in vitamin K deficiency and with warfarin (Coumadin) therapy; with deficiencies of multiple coagulation factors in liver disease or DIC; as an isolated coagulation factor deficiency in some patients with lupus anticoagulant (LA); and in patients exposed to topical bovine thrombin who develop antibodies to prothrombin (and not uncommonly to Factor V also). Bleeding manifestations depend on the level of prothrombin activity; usually no bleeding occurs with a prothrombin level >50% of normal.

Diagnosis

See Table 11–2 for the laboratory evaluation of the patient with a prothrombin (Factor II) deficiency.

Factor V Deficiency

Description

Factor V is a high-molecular-weight protein (approximately 300,000 Da) that acts as an accelerating cofactor for the enzymatic conversion of prothrombin to thrombin by Factor Xa. When Factor V is cleaved to Factor Va by thrombin, its cofactor activity is significantly increased. Factors Va and VIIIa are degraded by activated protein C. An isolated deficiency of Factor V is a rare cause of bleeding.

Apparent heterozygous and homozygous deficient states have been observed. Heterozygotes usually have levels of approximately 50% of normal and can experience bleeding or may be asymptomatic. In a cohort of 19 heterozygous patients, 50% had bleeding, with Factor V levels ranging from 21% to 55%. Homozygotes have variable levels below 50%; they are most likely to be symptomatic if the level is 10% or less.

As with the other coagulation factors, 2 major forms of the inherited deficiency are described: reduced or absent production of a normal Factor V molecule (absence form) and production of an abnormal molecule with reduced activity in normal amounts (dysfunctional form). A rare combined deficiency of Factors V and VIII is due to a genetic defect in intracellular transport of Factors V and VIII. Acquired deficiencies of Factor V occur with liver dysfunction or DIC.

Diagnosis

See Table 11–2 for the laboratory evaluation of the patient with a Factor V deficiency.

Factor VII Deficiency

Description

Factor VII is a vitamin K-dependent coagulation factor precursor that, when activated by thrombin, Factor Xa, or Factor IXa, is converted to Factor VIIa. This activated factor then converts phospholipid-bound Factor X into Factor Xa in the presence of calcium and tissue factor. It also converts Factor IX to Factor IXa. Factor VII deficiency may occur as an inherited or acquired disorder.

The inherited deficiency state, which is rare, may be present as reduced or absent production of a normal molecule (absence form) or production of an abnormal molecule with decreased activity in normal amounts (dysfunctional form). An inherited isolated deficiency of Factor VII in heterozygotes is usually associated with a Factor VII activity level of approximately 50%. In a cohort of 88 heterozygous patients, 36% had a bleeding tendency, with Factor VII levels ranging from 21% to 69%. In homozygotes, there are variable Factor VII activity levels below 50%. The bleeding risk is difficult to predict in these patients because the factor activity level correlates poorly with the patient's tendency to hemorrhage, but in general values <10% can be associated with major spontaneous bleeding. A large proportion of patients with less than 2% Factor VII do not bleed. Acquired Factor VII deficiency occurs in vitamin K deficiency and with warfarin therapy along with deficiencies of Factors II, IX, and X; and in DIC or liver disease along with multiple other coagulation factor deficiencies.

Intracranial hemorrhage has been reported in Factor VII-deficient patients, most often occurring in infants <1 year of age. Elevated Factor VII levels have been associated with an increased risk of cardiovascular disease.

Diagnosis

See Table 11–2 for the laboratory evaluation of the patient with a Factor VII deficiency.

Hemophilia A (Factor VIII Deficiency)

Description

Hemophilia A is a bleeding disorder resulting from a deficiency of Factor VIII procoagulant activity. Factor VIII circulates in the plasma bound to vWF. Approximately 90% of patients with hemophilia A synthesize low amounts of normal Factor VIII molecules, and 10% of patients with hemophilia A synthesize normal amounts of an abnormal (nonfunctional) Factor VIII. Hemophilia A is inherited as an X-linked trait, and 65% to 75% of patients have a positive family history. Disease prevalence in the United States is 1 in 10,000 males; the carrier state in females is rarely symptomatic. Hemophilia A and hemophilia B (Factor IX deficiency, see below) are clinically indistinguishable. The likelihood of hemorrhage depends on the amount of Factor VIII present; the majority of patients (approximately 50%-70% of hemophilia A patients) have severe disease. The severity of disease is categorized as follows:

• In mild disease: the VIII level is 6% to 20% of normal, with rare spontaneous bleeding.

• In moderate disease: the VIII level is 1% to 5% of normal, with occasional spontaneous bleeding.

• In severe disease: the VIII level is <1% of normal, with frequent spontaneous bleeding.

All hemophilia patients (A and B) may experience severe hemorrhage following trauma or surgery if there is no prior treatment to elevate the factor level. Bleeding that is characteristic of hemophilia (A and B) includes intra-articular (joint), intracranial, and intramuscular hemorrhage. The latter can produce a compartment compression syndrome. Easy bruising and prolonged bleeding after minor cuts and abrasions are also characteristic. The onset of hemorrhage is typically delayed following injury, and pathologic bleeding may occur hours after injury. Primary hemostasis (dependent on platelet plug formation) is intact, but secondary hemostasis (dependent on the fibrin clot generated by the coagulation cascade) is defective. Up to 15% of hemophilia A patients develop an inhibitor to Factor VIII at some time during the course of their disease (ie, an antibody against Factor VIII). The inhibitor develops only in those transfused with Factor VIII-containing products, and most often in patients with <1% Factor VIII. Factor VIII inhibitors may also spontaneously occur rarely in nonhemophiliacs (see the section “Factor VIII Inhibitors”).

Diagnosis

See Table 11–2 for information regarding the laboratory evaluation for Factor VIII deficiency.

Factor VIII Inhibitors

Description

Factor VIII inhibitors are antibodies, usually IgG, that bind to Factor VIII and inhibit its coagulant activity.

Factor VIII inhibitors have been found in several clinical situations.

• Inhibitors are diagnosed most commonly in patients with hemophilia A. Inhibitors occur in 10% to 15% of these patients and make the treatment of hemorrhage much more difficult. The vast majority of cases of Factor VIII inhibitors in hemophilia A patients occur in those with severe hemophilia A (<1% Factor VIII activity). Inhibitor formation is related to transfusion of exogenous Factor VIII, and usually develops before 100 treatment days if it appears. Two immune response patterns have been observed in hemophilia A patients. The first is a high response pattern. Inhibitors rise to a high titer in response to exposure to Factor VIII. The titer may not decline for months to years, even without further exposure to Factor VIII. Rapid anamnestic responses are often seen within 3 to 7 days of reexposure in these patients. In the second pattern, there is a low response. In addition, inhibitors usually remain at a low titer despite reexposure. They may occasionally disappear and reappear spontaneously. Little, if any, anamnestic response is likely found in a low responder.

• Spontaneous inhibitors to Factor VIII can occur in the postpartum patient. Usually they

• are recognized 2 to 5 months after the birth of the first child and disappear spontaneously after 12 to 18 months. However, the course is variable, and there are reports of death from hemorrhage in some patients. Antigenic differences between mother and fetus do not sufficiently explain the development of a Factor VIII inhibitor, and its cause remains unknown.

• Inhibitors may occur in those with allergic and enhanced immunologic reactions, including patients with:

  • (a) Rheumatoid arthritis

  • (b) Systemic lupus erythematosus

  • (c) Reactions to drugs, such as penicillin, chloramphenicol, sulfonamides, and phenytoin

  • (d) Malignancy

  • (e) Asthma

  • (f) Crohn disease

  • (g) Ulcerative colitis

  • (h) Pemphigus

  • (i) Multiple myeloma

• Inhibitors may appear in patients without any obvious underlying disorder. These are usually older individuals, and the inhibitor may remit in several months, persist for years, or disappear with immunosuppressive therapy.

In a hemophilia A patient, a poor response to treatment with Factor VIII concentrate may be the first indication that an inhibitor is present, or there may be an increased frequency of bleeding episodes. In nonhemophiliacs, development of a new hemorrhagic tendency is usually the presenting feature of a spontaneous Factor VIII inhibitor. The most favorable prognoses are for patients with low titer inhibitors, peripartum women, and patients without an underlying disorder.

Diagnosis

See Table 11–3 for information regarding the laboratory evaluation for a Factor VIII inhibitor.

Hemophilia B (Factor IX Deficiency)

Description

Hemophilia B is an inherited hemorrhagic disorder resulting from a lack of procoagulant activity of Factor IX. Factor IX is a vitamin K-dependent factor that, in its active form (Factor IXa), is a serine protease of the intrinsic pathway of the coagulation cascade. Approximately 70% to 90% of hemophilia B patients have a deficiency of a normal coagulant protein, and 10% to 30% produce an abnormal Factor IX that is nonfunctional. Inheritance is sex-linked, with affected males, and female carriers. Of hemophilia B patients, 60% to 70% have a positive family history for bleeding. The prevalence of hemophilia B is much less than that of hemophilia A. Approximately 1 in 50,000 males in the United States has hemophilia B versus 1 in 10,000 males with hemophilia A. The hemophilia B carrier state in the female is usually asymptomatic, as is the case with hemophilia A. Acquired Factor IX deficiency may occur along with deficiencies of Factors II, VII, and X in patients with vitamin K deficiency or those receiving warfarin therapy, and with deficiencies of other coagulation factors in patients with liver disease, DIC, or nephrotic syndrome.

As previously noted, hemophilia B is clinically indistinguishable from hemophilia A. The severity of hemorrhage depends on the amount of Factor IX activity present:

• In mild disease: 6% to 20% of normal IX activity is present, with rare spontaneous bleeding.

• In moderate disease: 1% to 5% of normal IX activity is present, with occasional spontaneous bleeding.

• In severe disease: <1% of normal activity is present, with frequent spontaneous bleeding.

Profuse bleeding may occur in any hemophilia B patient with trauma or surgery if there is no prior treatment to elevate the factor level. Bleeding in hemophilia B resembles that found in hemophilia A and includes deep tissue bleeding, intra-articular bleeding (hemarthrosis), intracranial bleeding (which may be lethal), and intramuscular bleeding with potential compartment compression syndrome. Severe mucosal membrane bleeding can occur in hemophilia, particularly after dental surgery.

Inhibitors develop to Factor IX in 1% to 5% of hemophilia B cases. These antibodies often occur in high titer and frequently present a major bleeding problem despite treatment.

Diagnosis

See Table 11–2 for information regarding the laboratory evaluation for patients with hemophilia B.

Factor X Deficiency

Description

An inherited isolated deficiency of Factor X is a rare disorder. Homozygotes and heterozygotes have been identified. Homozygotes usually possess <2% of normal activity. Heterozygotes usually possess 40% to 70% of normal activity. In a cohort of 15 heterozygous patients, 33% had a bleeding tendency, with Factor X levels ranging from 23% to 47%. Patients with Factor X values <10% can have a high risk of spontaneous major bleeding, and those with >40% are usually asymptomatic. Inherited Factor X deficiency, like the other factor deficiency states, occurs in 2 major forms: reduced or absent synthesis of a normal molecule (absence form) and synthesis of an abnormal molecule in normal amounts (dysfunctional form).

Acquired Factor X deficiency may result from warfarin or vitamin K deficiency (in the presence of deficiencies of Factors II, VII, and IX), from liver disease (with deficiencies of other factors synthesized in the liver), with DIC, or as an isolated deficiency in cases of amyloidosis. In amyloidosis, Factor X becomes irreversibly bound to amyloid fibrils in the extracellular space, and is thereby removed from the circulation.

Diagnosis

See Table 11–2 for information regarding the laboratory evaluation of patients with Factor X deficiency.

Factor XI Deficiency

Description

Factor XI deficiency is a commonly encountered disorder. Homozygotes typically have less than 20% of normal Factor XI activity. Heterozygotes have 20% to 70% of normal Factor XI activity. The deficiency in almost all cases appears to be a reduced or absent production of a normal molecule, rather than production of an abnormal or dysfunctional molecule.

The vast majority of the cases of Factor XI deficiency are in people of Jewish descent, particularly those of Ashkenazi origin. The frequency of the homozygous deficient state is 0.2% to 0.3% in the Ashkenazi population, and the frequency of the heterozygous state is extremely high, at approximately 5.5% to 11.0%.

The hemorrhagic tendency is variable for both heterozygotes and homozygotes. Patients with Factor XI levels <15% to 20% uncommonly have spontaneous bleeding but frequently have postoperative bleeding, and patients with levels between 20% and 65% tend to be asymptomatic or have low rates of postoperative bleeding. Bleeding does not correlate well with the level of Factor XI activity. Some homozygotes have an abnormal partial thromboplastin time (PTT), a very low Factor XI level of less than 10%, and no bleeding, even with surgery. The bleeding tendency of a particular individual is more closely related to the bleeding tendency of the patient's kindred than to the measured Factor XI level. The explanation, which is true for all mutations affecting coagulation factors, is that certain mutations produce a low level of Factor XI and a prolonged PTT but are not clinically significant in vivo. This is because they only affect the activity of the factor in the in vitro clotting factor assays, which are not exact replicas of clot formation in vivo. Acquired decreases in Factor XI can occur with pregnancy, proteinuria, liver dysfunction, and DIC.

Diagnosis

See Table 11–2 for information regarding the laboratory evaluation of patients with Factor XI deficiency.

Deficiencies of the Contact Factors

Description

The contact coagulation factors (so named because they were originally thought to activate the coagulation cascade by contacting the cut surface of the vessel wall) include Factor XII, PK, and HMWK. A deficiency of any of the contact factors prolongs the PTT because the PTT assay is constructed to involve these factors, even though the coagulation cascade in vivo does not depend on these factors. Bleeding diatheses have not been reported in patients with deficiencies at any level of Factor XII, PK, or HMWK. Factor XII deficiency is fairly common, with many thousands affected, especially individuals of Asian descent and children with tonsillitis. HMWK deficiency and PK deficiency are rare.

Diagnosis

See Table 11–2 for information regarding the laboratory evaluation for contact factor abnormalities.

Factor XIII Deficiency

Description

Factor XIII circulates in plasma as a zymogen and is converted to its active form (Factor XIIIa) by thrombin. Factor XIIIa catalyzes the formation of covalent bonds between chains of adjacent fibrin monomers. This stabilizes the fibrin clot, making it rigid and more resistant to the action of plasmin. Congenital deficiency of Factor XIII is rare. The bleeding tendency in homozygotes is characterized by umbilical stump bleeding in newborns (>90% of patients with clinically significant Factor XIII deficiency have this finding), intracranial hemorrhage, miscarriages, and posttraumatic hematomas, with bleeding often delayed hours to days after the trauma. Patients with mild or moderate deficiencies might have mucocutaneous bleeding or be asymptomatic. Patients with Factor XIII levels above 30% are usually always asymptomatic.

Diagnosis

See Table 11–2 for information regarding the laboratory evaluation for Factor XIII deficiency.

Antiplasmin Deficiency

Description

Antiplasmin or plasmin inhibitor (formerly known as alpha-2 antiplasmin) is a glycoprotein (GP) that serves as a regulator of fibrinolysis in several ways (see Figure 11–4). It blocks the enzymatic activity of plasmin (the major fibrinolytic enzyme) and other serine proteases, some of which are coagulation factors, and it inhibits the binding of plasminogen to fibrin. A bleeding diathesis is associated with the congenital deficiency of plasmin inhibitor. It is an extremely rare disorder and only homozygotes with <10% of normal plasmin inhibitor activity appear to be clinically affected. Those who do bleed may experience mucosal membrane bleeding (particularly in the genitourinary tract), subcutaneous hematomas, spontaneous bruising, and severe bleeding with trauma. Most heterozygotes are asymptomatic, but those few who are symptomatic have only a mild bleeding tendency. Acquired deficiency of plasmin inhibitor can occur in liver disease, nephrotic syndrome, amyloidosis, DIC, and, most notably, following thrombolytic therapy. In thrombolytic therapy, plasminogen is purposefully converted to plasmin, which results in the formation of plasmin–antiplasmin complexes, thereby reducing the amount of available antiplasmin.

Diagnosis

See Table 11–4 for information on the laboratory evaluation of plasmin inhibitor deficiency.

Vitamin K Deficiency

Description

In adults, vitamin K deficiency most often occurs secondary to disease or drug therapy; it rarely occurs as a dietary deficiency. Causes of vitamin K deficiency include:

• Warfarin therapy (reduces the amount of active vitamin K)

• Antibiotic therapy (capable of suppressing bowel flora that synthesize vitamin K)

• Malabsorption syndromes: cystic fibrosis, sprue, ulcerative colitis, Crohn disease, parasitic infections, short bowel syndrome, and ileojejunostomy (for morbid obesity)

• Dietary restriction with incidental decrease in vitamin K intake

• Long-term total parenteral nutrition

• Biliary obstruction

Vitamin K depletion can occur in as little as 2 weeks if both intake (enteral and parenteral) and endogenous production of vitamin K are eliminated. In early deficiency, Factor VII only, or Factors VII and IX only, may be decreased due to their shorter half-lives. Vitamin K deficiency may present as an asymptomatic prolongation of the PT in mild cases or as a major spontaneous hemorrhage in severe deficiencies. The degree of prolongation of the PT does not accurately predict the risk of hemorrhage.

Most antibiotics destroy bacterial flora and must be considered as a possible cause of vitamin K deficiency in the bleeding patient. However, certain cephalosporins produce vitamin K deficiency much more rapidly than other antibiotics. Cephalosporins with an N-methylthiotetrazole (MTT) group in position 3 directly inhibit the vitamin K-dependent carboxylase that is responsible for converting Factors II, VII, IX, and X to their active form. Cephalosporins in the MTT group include cefamandole, cefoperazone, cefotetan, moxalactam, cefmetazole, and cefmenoxime. Weekly prophylaxis with vitamin K has been recommended when MTT-cephalosporins are given to patients at high risk for vitamin K deficiency.

Diagnosis

See Table 11–5 for information on the laboratory evaluation for vitamin K deficiency.

Disseminated Intravascular Coagulation

Description

DIC is a common acquired coagulation disorder that occurs secondary to a variety of underlying disorders. The most common cause is infection; 10% to 20% of patients with gram-negative sepsis develop DIC. Other causes of DIC include obstetrical complications (retained dead fetus, placental abruption, amniotic fluid embolism, hypertonic saline-induced abortion, and septic abortion), extensive tissue injury (including trauma, ischemia, infarction, and burns), liver disease, transfusion of ABO-incompatible blood, and adult respiratory distress syndrome. The clinical presentation varies from an asymptomatic condition, detectable only by laboratory abnormalities, to a severe coagulopathy with a mortality of up to 80%.

The major events in acute DIC, independent of the cause, are microvascular thrombosis with consumption of platelets and coagulation factors, and then hemorrhage as a result of low levels of platelets and coagulation factors and overactivation of the fibrinolytic system to remove the thrombi. Hemorrhagic symptoms can include any of the following—petechiae, ecchymoses, mucosal oozing, hematuria, gastrointestinal tract bleeding, bleeding into surgical wounds, and prolonged bleeding at venous access sites. Severe bleeding may contribute to hypotensive shock.

DIC may present as a more chronic, low-grade condition in patients with malignancy. These patients are at risk for macrovascular (large vessel) thrombosis as well, most likely as a deep vein thrombosis.

The prolongations of the PT and the PTT reflect a decrease in fibrinogen and other coagulation factors that are consumed by clotting. In addition, fibrinogen is degraded by excess plasmin activation in the fibrinolytic system. Platelets are also consumed, and, therefore, the platelet count is typically low. The presence of FDP, 1 of which is the D-dimer, indicates that fibrin clots have been formed and subsequently degraded. There is no single laboratory test that can diagnose or exclude DIC, and the diagnosis is made when the characteristic laboratory abnormalities are present along with a known stimulus for DIC. A practical approach to diagnosis of DIC is to perform the PT, the PTT, and a D-dimer assay, with serial measurements of fibrinogen and platelets. In severe acute DIC, most of the laboratory test results will be abnormal, although fibrinogen may be normal or even elevated. In chronic DIC, the laboratory abnormalities may be less pronounced or even absent because the liver and bone marrow can increase production of coagulation factors and platelets, respectively, to offset the losses from consumption.

Diagnosis

See Table 11–6 for information on the laboratory evaluation for DIC.

Hemostatic Abnormalities in Liver Disease

Description

Patients with acute and chronic liver disease often have laboratory evidence of a hemostatic abnormality. These patients may be asymptomatic or have only mild bleeding problems, but those with advanced liver disease may experience a severe hemorrhage.

Hemorrhage in patients with liver disease may be due to 1 or more of the following:

• Coagulation factor abnormalities: These are caused by decreased hepatic synthesis of vitamin K-dependent factors (II, VII, IX, and X) and non-vitamin K-dependent factors. Decreased fibrinogen is usually found only in patients with severe hepatic failure; in fact, patients with acute hepatitis without hepatic failure usually have an increased fibrinogen level.

• Thrombocytopenia: This frequently occurs as a consequence of sequestration in the spleen, impaired platelet production, or increased platelet destruction. It is not usually a severe decrease in platelet number.

• Platelet dysfunction: The dysfunction is usually mild and its clinical significance is uncertain; platelet dysfunction may be clinically important only in liver disease patients with severe thrombocytopenia or severe renal failure, which can result in uremia-induced platelet dysfunction.

• DIC or a DIC-like syndrome: There is no general agreement as to whether the coagulation abnormalities that occur in patients with liver disease are due to DIC, liver disease alone, or a combination of these and other mechanisms. A DIC-like syndrome occurs frequently in patients with acute hepatic failure. Laboratory abnormalities in these cases include hypofibrinogenemia, thrombocytopenia, increased FDP such as D-dimer, and decreased levels of Factors V and VIII.

• Acquired dysfibrinogenemia (in patients with selected liver diseases [see the section “Fibrinogen Deficiencies”]): Impaired fibrin polymerization may result and thereby predispose the patient to bleeding.

• Increased fibrinolysis: Hemostatic abnormalities in patients with cirrhosis may be due to increased fibrinolysis. This may occur as a result of decreased hepatic clearance of plasminogen activators and decreased synthesis of inhibitors of fibrinolysis (see Figure 11–4).

Diagnosis

The laboratory evaluation for hemostatic defects from liver disease is shown in Table 11–7.

Immune Thrombocytopenic Purpura (ITP)

Description

ITP (where the I formerly stood for idiopathic) exists in both an acute and a chronic form. The disorder is one in which accelerated platelet destruction occurs in the absence of other causes such as DIC, thrombotic thrombocytopenic purpura (TTP), drug-induced thrombocytopenia, and neonatal thrombocytopenia. Platelet production is also often reduced.

The destruction of platelets in ITP is antibody-mediated. The amount of platelet-associated IgG is increased in the majority of patients with acute and chronic ITP. Many patients with chronic ITP have increased levels of antiplatelet antibodies in the serum, as well as on the platelet surface. It should be noted that there are a host of disorders unrelated to immune thrombocytopenias, which are associated with increased IgG on the platelet surface. Helicobacter pylori infection has been associated with ITP.

In acute ITP, the platelet may be an innocent target of an antipathogen antibody that cross-reacts with an epitope on the platelet membrane. Chronic ITP appears to be more of a classic autoimmune illness in which the target antigens for platelet autoantibodies are platelet GPs. Sequestration and destruction of antibody-coated platelets occur predominantly in the spleen.

Acute ITP usually presents as a childhood illness with peak incidence between 2 and 9 years. It is heralded by a prodromal illness, such as a viral respiratory infection, in 60% to 80% of cases. The infection occurs 2 to 21 days prior to onset of thrombocytopenia. The risk of hemorrhage is greatest during the first 1 to 2 weeks after the onset of acute ITP. Intracranial hemorrhage is the most feared complication of ITP. The majority of patients experience a spontaneous resolution of acute ITP 3 weeks to 3 months after onset. A small percentage of patients do not recover fully after 12 months, and advance to a diagnosis of chronic ITP.

Chronic ITP occurs most commonly between the ages of 20 and 50 years, and in females more often than in males (ratio of 2:1 to 3:1). It is characterized by the absence of a prodromal illness and the presence of mild bleeding that may continue for months before medical attention is sought. Manifestations include scattered petechiae or purpura, mostly on distal extremities, mild mucosal bleeding, easy bruising, epistaxis, and menorrhagia. ITP is often discovered in an asymptomatic patient found to have a low platelet count as part of a complete blood count (CBC). The diagnosis of ITP is made only after ruling out other causes for an isolated thrombocytopenia by history, physical examination, and laboratory studies.

Diagnosis

See Table 11–8 for information on the laboratory evaluation for ITP.

Drug-induced Immunologic Thrombocytopenia

Description

Many drugs have been implicated in drug-induced immune thrombocytopenia. However, most cases can be attributed to relatively few drugs, notably heparin, quinidine/quinine, gold salts, and sulfonamides. Exposure to most of these compounds is readily ascertained. However, when obtaining the patient's history, one should include inquiries regarding consumption of over-the-counter medications and topical medications, as well as soft drinks, mixers, and aperitifs to rule out exposure to quinine. The pathogenesis of thrombocytopenia for most drugs involves both the drug and IgG (as the predominant class of antibody involved). A plasma protein bound to the drug serves as the antigen; the antigen combines with a specific antibody, and this complex binds to the platelet membrane. This is known as an “innocent bystander” effect. The antibody-coated platelet is then sequestered and destroyed. Certain other drugs (eg, protamine, bleomycin, and ristocetin) can cause destruction of platelets by a direct toxic effect that is nonimmune. In heparin-induced thrombocytopenia (HIT), a complex of heparin and a circulating protein derived from the platelet, known as platelet factor 4 (PF4), acts as the antigen. The complex, along with bound antibody, binds to the platelet surface, causing platelet activation, and unlike other drug-induced thrombocytopenias, an increased risk for thrombosis.

The true incidence of drug-induced immunologic thrombocytopenia is not known. The incidence varies with the drug in question and the clinical condition or treatment of the patient. It may be as high as 1% to 3% of people exposed to the drug, as is the case with unfractionated (standard) heparin. Of quinidine users, approximately 1 in 1000 develop symptomatic thrombocytopenia. Drug-induced immunologic thrombocytopenia occurs most commonly in patients more than 50 years old, but it also has been reported in infants less than 1 year old. It is not possible to predict that patients will develop thrombocytopenia from drug treatment.

Ingestion of a drug that induces thrombocytopenia may produce flushing, fever, headache, and chills prior to onset of thrombocytopenia. The onset of thrombocytopenia may be abrupt following drug exposure or, if it requires antibody generation to lower the platelet count, it may be delayed for 4 to 15 days. Anamnestic responses may occur and if they arise, the delay is shorter. Bleeding may occur as early as 6 to 12 hours after exposure to the drug in highly responsive patients. Bleeding manifestations may include 1 or more of the following: petechiae, purpura (usually the first symptom), mucosal hemorrhagic bullae, gastrointestinal or genitourinary hemorrhage, intrapulmonary hemorrhage, and, lastly, intracranial hemorrhage, which is rare, but often lethal. HIT is unique in that bleeding is uncommon, and, as noted above, HIT patients are at risk for thrombosis rather than bleeding.

Diagnosis

See Table 11–9 for information on the laboratory evaluation for drug-induced immunologic thrombocytopenia. Laboratory tests for drug-induced thrombocytopenia are not routinely available, with the exception of testing for HIT. If HIT is considered, a platelet count should be performed first. If the platelet count decreases to 50% or less of its apparent baseline value, a test for antibodies to the heparin–PF4 complex or a functional test that shows platelet activation in the presence of heparin and the patient's plasma should be performed.

Posttransfusion Purpura

Description

Posttransfusion purpura (PTP) is a rare syndrome characterized by the sudden onset of thrombocytopenia 7 to 10 days following transfusion of blood or blood products containing platelets or platelet material. The thrombocytopenia appears to be due to antibody-mediated destruction of autologous as well as transfused platelets. In over 90% of cases, the antibody that develops in the affected individuals is directed against the antigen HPA-1a, formerly known as P1^A1, on platelet membrane GP IIIa. In these cases, the recipient's own platelets are HPA-1a negative. It is not known why there is destruction of the patient's own HPA-1a-negative platelets following platelet transfusion with HPA-1a-positive platelets. Only 2% to 3% of the population in the United States lacks this antigen. Antibodies against other platelet-specific antigens have been reported in PTP, but they are much less commonly encountered. In almost all cases, the development of anti-HPA-1a antibody is thought to be an anamnestic response, with prior sensitization occurring through previous transfusion or pregnancy.

PTP occurs predominantly in females, perhaps due to the likelihood of sensitization through pregnancy. The interval between the first exposure to the HPA-1a antigen and the transfusion that incites the thrombocytopenia is greater than 3 years in most cases in which the information has been reported. The onset of thrombocytopenia is fulminant in most cases, with the platelet count decreasing to <10,000/μL. Hemorrhage usually begins with purpura and mucocutaneous bleeding, and may progress to gastrointestinal and genitourinary bleeding, epistaxis, oozing from intravenous access sites, and intracranial hemorrhage. In severely affected patients sustained with supportive therapy of red blood cells and/or platelets, the thrombocytopenia usually begins to resolve in 14 days (mean value with a range 1-35 days). Cases with less severe thrombocytopenia apparently require a longer recovery period (24 days average, range 6-70 days). The outcome is fatal in approximately 10% of cases, usually due to hemorrhage. The risk of fatal hemorrhage appears to be greatest at the onset of the disease. Recurrent PTP has been documented, with the recurrence appearing no sooner than 3 years after the first episode, even though antibody may persist in the patient's blood during the intervening time and there may be repeated challenges with exogenous platelets.

Diagnosis

See Table 11–10 for information on the laboratory evaluation for PTP.

Neonatal Alloimmune Thrombocytopenia (NAIT)

Description

NAIT is a disorder in which there is destruction of platelets in the fetus and newborn. The destruction occurs following transplacental passage of maternal IgG antibodies directed against a platelet-specific antigen present on fetal platelets and absent from the mother's platelets. The antibody-coated platelets are removed from the circulation by the neonate's reticuloendothelial system around the time of birth. The estimated incidence ranges from 1 in 5000 to 1 in 2000 births, with an increasingly higher incidence in recent years attributed to improved surveillance and serologic testing for the disorder. The platelet-specific antigen implicated in 80% to 90% of all cases (and 95% of symptomatic cases) of NAIT is HPA-1a. As previously noted, this antigen is present on the platelets of 97% to 98% of the general population. In approximately 50% of cases, NAIT occurs during the first pregnancy; when it does occur, there is a 97% chance that the next pregnancy will be affected.

Affected newborns are usually the product of an otherwise uncomplicated pregnancy and delivery. Within hours after birth, petechiae and ecchymoses appear in a generalized distribution. Other clinical signs include neurologic abnormalities if intracranial hemorrhage occurs, and pallor from anemia, if the bleeding is severe. Intracranial hemorrhage is the leading cause of death in NAIT, with a 50% mortality. Overall mortality from NAIT is approximately 5% to 10%. Thrombocytopenia usually persists for approximately 2 weeks in untreated cases (range of 1 week to 2 months), and 1 week in treated cases.

Diagnosis

See Table 11–11 for information on the laboratory evaluation for NAIT.

Discussions of TTP and hemolytic–uremic syndrome (HUS), which are thrombocytopenias associated with thrombosis, are presented among the thrombotic disorders.

Essential Thrombocythemia

Description

Essential thrombocythemia is a chronic myeloproliferative disorder, characterized by thrombocytosis arising from the clonal proliferation of a neoplastic multipotent stem cell. Life expectancy can be essentially normal with a median survival of 10 to 15 years, but the disease course is frequently complicated by both hemorrhage and thrombosis. A small percentage (<5%) of patients progress to acute leukemia, predominantly those patients previously treated with radioactive phosphorus or alkylating agents to reduce their platelet counts. At the time of diagnosis using older criteria, mild splenomegaly occurs in 30% to 50% of patients, and hepatomegaly in 15% to 20%. Using 2008 WHO diagnostic criteria, splenomegaly is present in only a minority of patients at diagnosis. The incidence of the disorder is higher in older age groups.

The principal diagnostic feature of essential thrombocythemia is a persistently elevated platelet count with bone marrow megakaryocyte hyperplasia. Patients with this disorder can progress to a “spent” phase, characterized by myelofibrosis and a low platelet count. The purpose of the laboratory testing is to eliminate other possible etiologies for the thrombocytosis. Other entities in the differential diagnosis of an elevated platelet count include reactive thrombocytosis and other myeloproliferative disorders—myelofibrosis, polycythemia vera, and chronic myelogenous leukemia.

Diagnosis

See Table 11–12 for information on the laboratory evaluation for essential thrombocythemia.

von Willebrand Disease

Description

vWD is caused by a quantitative deficiency of normal vWF in the majority of cases and a qualitatively abnormal vWF in the remainder of cases. vWF normally polymerizes to form multimers, which are aggregates of a single vWF polypeptide; in normal plasma, the multimers have a range of sizes. vWF has 2 major roles:

• Platelet adhesion: Large multimers of vWF (ie, those with many units of the single polypeptide) effectively promote platelet adhesion to the subendothelium in injured vessels; if only small multimers are present, platelet plugs form poorly.

• Binding of Factor VIII: vWF circulates in the plasma with Factor VIII, the coagulant protein that is lacking in hemophilia A. vWF prolongs the half-life of Factor VIII by protecting it from rapid degradation. If vWF is reduced, Factor VIII coagulant activity is often reduced as well.

vWD prevalence estimates vary, with reported values as high as 1% of the general population. Unlike hemophilia A and B, vWD affects both men and women. It is likely to be the most common inherited bleeding disorder. There are 3 major types of vWD. The types were reorganized and renumbered with Arabic numerals in the 1990s (see Table 11–13). The most common type (type 1) is usually a mild bleeding disorder; it accounts for the majority of all cases of vWD. Type 2 vWD includes patients with qualitative vWF defects. Type 3 is rare and inherited as an autosomal recessive trait. It is associated with severe bleeding and very low to absent vWF levels. The types are distinguished from each other by laboratory testing.

It should also be noted that the mean vWF levels vary with blood type as shown in the second portion of Table 11–14.

More than 65% of patients with vWD have type O, presumably because patients with this blood type start from a lower baseline value for vWF.

The severity of bleeding is highly variable among patients, even within the same subtype of vWD, and even within an individual patient over time. Typically, bleeding manifestations such as easy bruising or epistaxis begin in early childhood. Other manifestations include menorrhagia and mucous membrane bleeding (from the gingiva, oropharynx, and gastrointestinal and genitourinary tracts). Profuse hemorrhage may occur with a significant hemostatic challenge such as trauma or surgery.

Diagnosis

Laboratory test results vary with the type and subtype of vWD. Like the severity of bleeding, the laboratory values can also vary widely over time for an individual patient, and may sometimes be normal. Normal results from a single determination do not rule out the diagnosis. If the patient history strongly suggests vWD, and the test results are normal, the tests should be repeated at a later time because plasma levels for vWF are increased during pregnancy, stress, while receiving oral contraceptives, and during an acute illness or injury. Therefore, values obtained at these times may be unreliable for diagnosis. It is also not yet clear if the absolute level of vWF or the level relative to the mean vWF for the blood type of the patient is more important in establishing the diagnosis. Current guidelines note that the absolute level of vWF seems to be more important than the level relative to blood type.

See Table 11–14 for information on the laboratory evaluation for vWD.

von Willebrand Disease Types, Subtypes, and Their Expected Test Results

• Type 1: vWF multimers of all sizes are decreased due to a defect in synthesis or release of vWF from the endothelium, the site of most vWF synthesis. Functional (ristocetin cofactor or vWF:RCo) and antigenic (vWF antigen or vWF:Ag) levels of vWF are usually proportionately decreased. Factor VIII activity might also be low. The vWF multimer pattern shows a normal distribution of multimers.

• Type 2A: Absence of large and intermediate-size vWF multimers from the plasma and platelet surface, due to a defect in the synthesis or polymerization of multimers, or from increased proteolysis of multimers. Functional activity (vWF:RCo) is decreased compared with antigenic levels (vWF:Ag). Therefore, vWF:RCo < vWF:Ag < Factor VIII is the most commonly observed pattern in type 2A. The vWF multimer pattern shows an abnormal distribution of multimers, with the absence of large and intermediate-size multimers.

• Type 2B: Marked deficiency of large vWF multimers from plasma. Intermediate-size and small multimers are present. Large multimers are present on the patient's platelets, due to increased affinity of the abnormal vWF molecule for the platelet surface. Functional and antigenic levels in plasma samples are similar to those in type 2A (vWF:RCo < vWF:Ag < Factor VIII). The vWF multimer pattern shows the absence of large multimers from plasma. The patient's platelets show increased aggregation at low concentrations of ristocetin that do not cause normal platelets to aggregate. The patient's platelets aggregate at low concentrations of ristocetin because they are coated with large vWF multimers.

• Types 2M and 2N are uncommon and are briefly described in Table 11–13.

• Type 3: Severe deficiency of all vWF multimers, due to a marked defect in synthesis. Factor VIII activity is less severely affected than vWF activity. Both functional and antigenic vWF levels are markedly reduced. The vWF multimer pattern shows a virtual absence of all-size multimers.

• Platelet-type Willebrand disease: vWF is qualitatively normal, but abnormal platelets have an increased affinity for large multimers of vWF due to a defect in platelet GP Ib. The laboratory test values are similar to those in type 2B.

• Acquired vWD: This disorder has been found in patients with systemic lupus erythematosus, multiple myeloma, Waldenström macroglobulinemia, lymphoproliferative disorders, and other diseases. Patients have no congenital or familial history of bleeding. Causes of the decrease in circulating vWF include adsorption of large multimers onto cells (eg, lymphocytes or tumor cells) or the presence of antibodies to vWF. Acquired vWD resolves when the underlying disorder is effectively treated.

Bernard–Soulier Disease and Glanzmann Thrombasthenia

Description

Bernard–Soulier syndrome (BS) and GT are rare congenital hemorrhagic disorders that result from absent or defective specific platelet membrane GPs, impairing platelet function. BS is characterized by a decrease of functional GP Ib/IX/V, the platelet receptor for vWF. GT is characterized by a decrease of functional GP IIb/IIIa, the complex that mediates platelet aggregation by binding fibrinogen to the platelet surface when platelets are activated.

GT often decreases in severity with age. Manifestations include easy bruising, epistaxis, mucous membrane bleeding—particularly in the gastrointestinal tract—and menorrhagia. The amount of hemorrhage is highly variable among affected patients.

Diagnosis

See Table 11–15 for information on the laboratory evaluation for BS and GT.

Platelet Storage Pool Disease

Description

Platelet SPD represents a group of disorders in which there is a deficiency of platelet granules. Decreased secretion of platelet granular contents at the time of platelet activation makes the platelets less hemostatically effective. The congenital forms of SPD include:

• Delta SPD: platelets have a decreased number of delta (dense) granules; these secretory granules contain ADP, polyphosphate, serotonin, and calcium.

• Alpha-delta or alpha-partial delta SPD: decreased number of delta granules with either a complete or partial deficiency of alpha granules; alpha granules contain many proteins including fibrinogen, PF4, platelet-derived growth factor, and beta-thromboglobulin.

• Alpha SPD (“gray platelet syndrome”): decreased number of alpha granules, and a normal number of delta granules; platelets appear gray, large, and vacuolated on a peripheral blood smear.

SPD also may occur as an acquired abnormality, acutely in patients who have been supported on a cardiopulmonary bypass device and chronically in some cases of acute leukemia and myeloproliferative disorders. The molecular basis of most types of congenital SPD is unknown. It may result from abnormal granule morphogenesis or abnormal granule maturation in megakaryocytes. SPD may be a manifestation of a global defect in granule formation as in the Hermansky–Pudlak syndrome (see below). Hereditary SPD is the most common congenital qualitative platelet disorder, but it is still quite rare.

Most patients with SPD have mild bleeding symptoms. Bleeding manifestations of SPD include mild mucous membrane bleeding, easy bruising, menorrhagia, and excessive bleeding following dental or general surgery. SPD may also occur as a component of the following syndromes:

• Hermansky–Pudlak syndrome: Features include delta SPD, oculocutaneous albinism, pulmonary fibrosis, and the accumulation of ceroid-like material in cells of the reticuloendothelial system. One subtype is due to a defective gene (called HSP1) on chromosome 10.

• Chediak–Higashi syndrome: Features include delta SPD with giant platelet granules, photophobia, nystagmus, pseudoalbinism, lymphadenopathy, splenomegaly, and increased susceptibility to infection. It is attributed to defects in a gene called CHS1 on chromosome 1, affecting protein trafficking.

• Thrombocytopenia with absent radius: Features include alpha SPD and absence of the radius bone.

• Wiskott–Aldrich syndrome: Features of this X-linked recessive disorder include delta SPD with other metabolic platelet defects, recurrent infections, eczema, lymphocytopenia, multiple cellular and humoral immunologic defects, and thrombocytopenia with microplatelets (small platelets); the thrombocytopenia may resolve following splenectomy. It is attributed to a genetic defect in a gene called WASP on the X chromosome, affecting signal transduction and other functions.

Diagnosis

See Table 11–16 for information on the laboratory evaluation for storage pool deficiency.

Hemostatic Defects in Uremia

Description

The bleeding tendency in uremia-induced hemorrhage is attributed to platelet dysfunction and endothelial cell dysfunction.

Bleeding manifestations may be mild or severe and can include petechiae, ecchymoses, epistaxis, and purpura. Paradoxically, chronic renal failure is also associated with an increased incidence of arterial and venous thrombosis, and, therefore, can influence hemostasis toward bleeding or clotting.

Diagnosis

See Table 11–17 for information on the laboratory evaluation for hemostatic defects in uremia.

Drug-induced Qualitative Platelet Dysfunction

Description

Platelet dysfunction may occur on ingestion of a wide variety of drugs, particularly aspirin and clopidogrel (Plavix). Due to the ubiquity of aspirin in over-the-counter medications, many medications are implicated in platelet dysfunction. Some patients consume multiple drugs, such as aspirin and clopidogrel, with different and additive antiplatelet effects and thereby inhibit platelet function by more than 1 mechanism. Drug-induced platelet dysfunction can present a high bleeding risk in patients with existing hemostatic defects, but typically does not result in clinically significant bleeding in normal individuals. When hemorrhage does occur, there is usually an underlying hemostatic disorder affecting either the platelets or coagulation factors, or an anatomic lesion, such as an ulcer, that predisposes the patient to bleeding.

Commonly encountered coagulopathies that place patients at risk for bleeding when there is a superimposed drug-induced platelet defect include vWD, thrombocytopenia of any cause, and anticoagulation therapy. Hemorrhagic manifestations can include petechiae and purpura, ecchymoses, mucosal membrane bleeding, hematuria, epistaxis, and oozing from intravenous access sites and surgical incisions.

Diagnosis

Laboratory tests are of little value in predicting the clinical significance of drug-induced platelet defects. They can confirm the presence of abnormal platelet function, but cannot assess the risk of bleeding. Furthermore, laboratory abnormalities in platelet function are not specific for a particular drug. See Table 11–18 for information on the laboratory evaluation for drug-induced platelet defects.

In this chapter, the disorders associated with thrombosis (Figure 11–5) are grouped into those with a relatively higher prevalence and those with a relatively lower prevalence. Among those with a higher prevalence is activated protein C resistance, which is produced by the Factor V Leiden mutation. This mutation is present in 3% to 5% of Caucasian populations. Other thrombotic disorders with a high prevalence are the prothrombin G20210A mutation, and the antiphospholipid antibody syndrome (an acquired disorder). The thrombotic disorders with a lower prevalence include protein C deficiency, protein S deficiency, and antithrombin deficiency. Elevated plasma homocysteine levels may also increase the risk for thrombosis. Plasminogen deficiency is also rare, and its association with thrombosis is controversial. Two other rare conditions, dysfibrinogenemia of certain types and essential thrombocythemia, can produce either thrombosis or bleeding. Also rare are TTP and HUS.

Hypercoagulable States

Description

Hypercoagulable states are associated with an increased risk for thrombosis (Table 11–19). There are both hereditary and acquired hypercoagulable states. Hereditary forms may arise from a quantitative or qualitative deficiency of a regulatory anticoagulant protein, such as protein C, protein S, or antithrombin (see Figures 11–2 and 11–3). Activated protein C resistance is caused by a mutation in the Factor V molecule (nearly always the Factor V Leiden mutation), which prevents activated protein C-mediated inactivation of Factor Va. The prothrombin G20210A mutation is prevalent in Caucasian populations, similar to Factor V Leiden. Prothrombin G20210A is a mutation in the promoter of the prothrombin gene, causing increased synthesis of prothrombin (Factor II). Hyperhomocysteinemia, a disorder in which there is an abnormally high level of plasma homocysteine, can be hereditary or acquired. Hyperhomocysteinemic individuals are at increased risk for coronary artery disease and deep venous thrombosis. However, studies have not yet shown that reducing slightly or moderately elevated homocysteine with vitamin therapy reduces the thrombotic risk. A block in the pathway at any 1 of the several steps results in the accumulation of homocysteine. When the homocysteine level is manyfold higher than the upper limit of normal, this very high value may contribute to the damaging effects on the blood vessel wall. Acquired hypercoagulable states arise from a diverse array of clinical conditions. They include malignancy, immobilization, surgery, trauma, obesity, smoking, infection, prolonged travel, and the use of oral contraceptives, estrogen replacement therapy, and progesterone, among many others. An acquired hypercoagulable state for which specific coagulation testing is available is the antiphospholipid antibody syndrome.

The presence of 1 hypercoagulable condition alone is not usually sufficient to initiate thrombosis. The presence of a second (or more), superimposed hypercoagulable condition (often called a “second hit”) appears to be required to provoke a thrombotic event. For example, a person with activated protein C resistance may not experience a thrombotic event until suffering major trauma as a “second hit.”

Diagnosis

Laboratory testing for hypercoagulable states is most often performed for patients presenting with a personal or family history of thrombosis. The laboratory evaluation for hereditary hypercoagulable states is best performed as a panel of test results. The most common disorders are activated protein C resistance (caused nearly always by Factor V Leiden) and prothrombin G20210A, and the less common are deficiencies of protein C, protein S, and antithrombin. Frequently, antiphospholipid antibody testing is performed in conjunction with the tests for hereditary hypercoagulable disorders. If all of these tests are negative and the clinical suspicion for a congenital hypercoagulable state remains high, additional testing for the more rare hypercoagulable disorders can be performed. There are many acquired conditions or treatments that reduce the level of protein C, protein S, and antithrombin in the test panel for hypercoagulability, but despite this, the risk for thrombosis is not significantly increased on that basis. For example, warfarin reduces the levels of protein C and protein S but reduces thrombotic risk. Antithrombin is lowered by heparin therapy. Pregnancy, oral contraceptives, and estrogen therapy can all decrease the activity of protein S, although they can induce a hypercoagulable state by other mechanisms. Active clot formation, liver dysfunction, or DIC can lower protein C, protein S, and antithrombin. In situations where such a confounding variable exists that alters the level of protein C, protein S, or antithrombin, the tests should be repeated (if possible) when the variable is no longer present to obtain the patient's true baseline values. This should allow determination of whether a heritable deficiency of any of these 3 proteins truly exists:

• Activated protein C resistance: Usually, the first assay performed is a functional assay for activated protein C resistance. If the result is abnormal, genetic analysis is performed, to confirm whether Factor V Leiden is present in the heterozygous state, present in the homozygous state, or absent.

• Prothrombin G20210A: This mutation is identified by genetic analysis that can specifically identify heterozygous and homozygous states.

• Hyperhomocysteinemia: Plasma homocysteine levels are measured by a variety of automated methods.

• Protein C, protein S, and antithrombin deficiencies: Individual functional assays to measure the activity of these endogenous anticoagulant proteins detect both qualitative (normal number of abnormally functioning molecules) and quantitative (low number of normally functioning molecules) deficiencies. In contrast, an antigenic (immunologic) assay, which measures only the quantity of protein present, can detect quantitative but not qualitative deficiencies. Therefore, the first assay performed should be a functional assay. If the result is abnormal, an antigenic assay can be performed to determine if the cause of the decreased activity is a quantitative or qualitative deficiency of the protein.

• LA (an antiphospholipid antibody): A variety of tests can be used to detect an LA. This antibody interferes with the cofactor action of phospholipid in the coagulation cascade in laboratory assays only (see the section “Antiphospholipid Antibodies: The Lupus Anticoagulant, Anticardiolipin Antibodies, and Beta-2 Glycoprotein I Antibodies” [Beta-2 GP I antibodies]). Various phospholipid-dependent coagulation test times, especially PTT-based or dilute Russell viper venom time (DRVVT) assays, can be used. The DRVVT is the clotting time obtained using Russell viper venom, which contains a Factor X activator.

• Anticardiolipin antibody or beta-2 GP I antibody (both are antiphospholipid antibodies): These antiphospholipid antibodies may or may not be associated with the presence of an LA (see the section “Antiphospholipid Antibodies: The Lupus Anticoagulant, Anticardiolipin Antibodies, and Beta-2 Glycoprotein I Antibodies”), and they are detected by enzyme-linked immunoassays.

See Table 11–19 for characteristics of the hypercoagulable states.

Antiphospholipid Antibodies: The Lupus Anticoagulant, Anticardiolipin Antibodies, and Beta-2 Glycoprotein I Antibodies

Description

Antiphospholipid antibodies recognize specific phospholipid–protein complexes rather than phospholipid alone. They can be immunoglobulin type IgG or IgM, or, less commonly, IgA. The LA is an immunoglobulin that can interfere with phospholipid-dependent coagulation reactions in laboratory assays without inhibiting the activity of any specific coagulation factor. It targets phospholipids bound to prothrombin, beta-2 GP I, protein C, protein S, or other proteins bound to phospholipids. Anticardiolipin antibodies are another type of antiphospholipid antibody, which target beta-2 GP I bound to a particular phospholipid, cardiolipin; these can be detected by anticardiolipin antibody immunoassays. Anti-beta-2 GP I antibodies also target beta-2 GP I.

An antiphospholipid antibody may occur in apparently healthy individuals with no detectable illness. It also occurs in patients with a variety of clinical conditions or disorders including:

• Systemic lupus erythematosus and other autoimmune disorders

• Malignancy

• Following ingestion of selected drugs (procainamide, quinidine, phenytoin, chlorpromazine, valproic acid, amoxicillin, augmentin, hydralazine, streptomycin, and propranolol have all been reported to induce an LA)

• Infectious diseases—bacterial (including spirochetal and mycobacterial), viral, fungal, and protozoal infections

• Following vaccination

Estimates of prevalence have been highly variable because results are completely dependent on the test(s) used for detection of antiphospholipid antibodies, and some methods are more sensitive than others. Approximately 2% of patients with a prolonged PTT will have an LA as the cause of the prolongation. LA is the most common cause of a prolonged PTT that remains prolonged in a PTT mixing study (a PTT performed on a sample of mixed patient and normal plasma). It is estimated that 1% to 5% of the general population has an antiphospholipid antibody. The frequency of antiphospholipid antibody in systemic lupus erythematosus patients is in the 30% to 40% range. Antiphospholipid antibodies due to infections are typically transient and asymptomatic.

Although the LA acts as an anticoagulant in vitro, it does not appear to be associated with hemorrhage, even with surgical challenge. Rare cases of bleeding in patients with the LA can almost always be attributed to specific abnormalities in hemostasis that happen to be present along with the LA. Decreased prothrombin (Factor II) is occasionally found with the LA. The LA can bind directly to prothrombin, but typically the LA does not neutralize the procoagulant activity of prothrombin even when it does bind to it. In an occasional patient, however, antibody binding does reduce the prothrombin concentration by accelerated clearance of prothrombin/antiprothrombin complexes, and these patients can have hemorrhagic complications. Concomitant thrombocytopenia is not infrequently found in patients with the LA, and this also may be a cause for hemorrhage.

Antiphospholipid antibodies are associated with an increased risk for venous and arterial thrombosis. The role of the antiphospholipid antibody in thrombosis is not clear, although several mechanisms for thrombosis have been proposed. The incidence of clinically apparent thromboembolism in patients with the LA, with or without systemic lupus erythematosus, is difficult to determine because of the variety of tests used to detect the LA. However, data suggest that the percentage of patients with the LA who will develop thrombosis is 1% per year if there is no history of thrombosis and 5.5% per year if there has been at least 1 prior thrombotic event. High titers of anticardiolipin or anti-beta-2 GP I antibodies present a higher risk for complications than low titers, and IgG is thought to be higher risk than IgM or IgA. There is a greater risk for thrombosis if more than 1 of the 3 antiphospholipid antibody tests (LA, anticardiolipin antibodies, and anti-beta-2 GP I antibodies) are abnormal.

Recurrent spontaneous abortion has been reported to be increased in patients with antiphospholipid antibodies. There is evidence suggesting that thrombosis and infarction in the placenta mediate antiphospholipid antibody-associated spontaneous abortion in a significant number of women experiencing recurrent fetal loss or premature birth.

Diagnosis

There are no “gold standard” tests that unequivocally establish the presence of antiphospholipid antibodies:

• For the LA: A prolonged PTT and a PTT mixing study that does not correct into the normal range are clues to the presence of the LA, although some PTT reagents are not sensitive to the LA. Therefore, it should be noted that the routine PTT is not an appropriate screening test for the LA. A PTT- or DRVVT-based test with a reduced amount of phospholipid can be used as a screening test for the LA. If it is prolonged, a 1:1 mixing study and a confirmatory assay should be performed. In the presence of an LA, the PTT or DRVVT usually remains prolonged when equal portions of patient plasma and normal plasma are mixed. Confirmatory assays demonstrate that the clotting time shortens toward normal when excess phospholipid is added, overcoming the LA effect. The PT is not typically increased by the LA, unless the patient has an associated hypoprothrombinemia or the thromboplastin used in the PT is one that is particularly sensitive to inhibition by the LA.

• For anticardiolipin or anti-beta-2 GP I antibodies: An enzyme-linked immunosorbent assay (ELISA) is used that quantitates IgG and IgM antibody levels in arbitrary units. IgA is also measured in some kits.

Patients with antiphospholipid antibodies may have a false-positive serologic test result for syphilis (such as Venereal Disease Research Laboratories [VDRL] and rapid plasma reagin [RPR]).

See Table 11–19 for information on the laboratory diagnosis of antiphospholipid antibodies.

Thrombotic Thrombocytopenic Purpura

Description

TTP is a clinical syndrome that is characterized by a triad (1-3 below, more commonly) or pentad (1-5 below, less commonly) of signs and symptoms:

1. Thrombocytopenia with generalized purpura, and mucous membrane bleeding

2. Hemolytic anemia (microangiopathic) sufficient to cause jaundice or pallor

3. Neurologic abnormalities, which may include fluctuating weakness, dysphagia, headache, dementia/behavioral changes, obtundation, seizures, diplopia, paresthesias, and coma

4. Fever

5. Renal dysfunction, which may include hematuria, proteinuria, or renal insufficiency

The characteristic, but not pathognomonic, pathology includes platelet and fibrin “hyaline” thrombi in the small arteries, arterioles, and capillaries in a widespread organ distribution. Organ ischemia and infarction that arise from these thrombi are thought to give rise to the observed fever and organ dysfunction. The etiology for TTP has been shown to be a deficiency of vWF-cleaving protease. Nonfamilial cases of TTP are a result of an inhibitor to the vWF-cleaving protease; the familial form of TTP is apparently caused by a constitutional deficiency of the protease. The unusually large forms of vWF in the plasma of patients with TTP promote the aggregation of platelets in vivo, which accounts for most of the clinical findings.

TTP can occur at any age but is most common between the ages of 20 and 50 years. Peak incidence is in the third decade. There is a female to male ratio of 3:2. TTP usually occurs as an acute, fulminant disease, but may also occur in a chronic form or in an acute relapsing form. The acute and acute relapsing types are often preceded by a viral prodrome. Nonspecific signs such as malaise, weakness, fatigue, and anorexia may predominate at first, until the above triad or pentad develops in days to weeks. The chronic type usually pursues an indolent, low-grade course, with ongoing disease activity for months.

Nonspecific manifestations in other organ systems resulting from ischemia may include:

• Cardiac: conduction defects, sudden death, and heart failure

• Pulmonary: lung infiltrates and acute respiratory failure

• Gastrointestinal: abdominal pain due to visceral ischemia, pancreatitis, and gastrointestinal mucosal hemorrhage

Diagnosis

See Table 11–20 for information on the laboratory evaluation for TTP.

Hemolytic–Uremic Syndrome

Description

HUS is a clinical syndrome with presentation and manifestations similar to TTP. Despite the clinical similarity, evidence has shown that the low levels of vWF-cleaving protease found in TTP are not found in HUS. Thus, the pathogenesis of these 2 disorders is apparently completely different. HUS is characterized by fever, microangiopathic hemolytic anemia, thrombocytopenia, and renal dysfunction. It differs clinically from TTP in the following ways: neurologic symptoms are less pronounced or absent in HUS; renal function is usually more impaired in HUS than in TTP; HUS occurs in a younger population than TTP with a peak incidence between 6 months and 4 years, with males and females equally affected; HUS is a more common entity than TTP; as with TTP, hyaline thrombi may be found, but in most cases they tend to be confined to the glomerular capillaries and afferent arterioles.

HUS occasionally occurs in adults, and is often distinguished from childhood HUS because of its strong association in adults with obstetrical complications such as eclampsia. The prognosis is worse in adults than in affected children, with an adult mortality as high as 60%.

Acute HUS occurs in nonfamilial and familial forms, which may have different causes. Nonfamilial forms are most often associated with a diarrheal illness caused by a Shiga-toxin-producing E. coli (in particular, E. coli O157:H7). Familial forms have been linked to abnormalities in complement factor H and factor I. The majority of childhood cases resolve without sequelae, if children with acute renal failure receive dialysis when necessary. The prognosis depends on the duration of renal failure and the severity of the neurologic disturbance. Renal function returns to normal in approximately 90% of childhood cases.

Diagnosis

See Table 11–21 for information on the laboratory evaluation for HUS.

Anticoagulant Therapies

Thrombosis can be treated and/or prevented with anticoagulant therapies, which are therapies that inhibit the coagulation cascade, thereby inhibiting fibrin clot formation. Since too little anticoagulation might allow new thrombosis to occur while too much anticoagulant can cause bleeding, laboratory tests can be used to determine how much anticoagulation is present in the patient, so that the dose can be adjusted if needed. For decades, heparin and warfarin were the only anticoagulants available for use. Heparin is given intravenously or subcutaneously, and warfarin is taken orally. As described previously, heparin inhibits multiple coagulation factors, and the effect of this inhibition is that heparin prolongs the PTT. Most PT reagents contain a heparin neutralizer that largely prevents significant PT prolongations for patients on heparin. Warfarin decreases the synthesis of active forms of Factors II, VII, IX, and X, which prolongs the PT. Different PT methods can demonstrate different degrees of prolongation with warfarin. Therefore, a formula is used to convert the PT into a more standardized number that takes into account the relative sensitivity of the PT reagent. This calculation is called the international normalized ratio (INR), and it is meant to provide similar results no matter what method is in use by the laboratory. To be sure the correct amount of anticoagulation is present, heparin is closely monitored with the PTT, and warfarin is closely monitored with INR measurements.

Newer anticoagulants have emerged that do not routinely need monitoring with laboratory tests. Nevertheless, laboratory monitoring may still be occasionally indicated. Low-molecular-weight heparin, fondaparinux, rivaroxaban and apixaban inhibit Factor Xa. Therefore, these anticoagulants can be monitored with anti-Factor Xa assays. These are assays that can measure the concentration of the anticoagulant based on how much inhibition of Factor Xa is detected. Direct thrombin inhibitors (argatroban, bivalirudin, and dabigatran) inhibit thrombin, which prolongs the PTT and to a lesser extent the PT. A dilute thrombin time assay has been shown to have a linear relationship to therapeutic doses of dabigatran. The PTT can be useful to monitor argatroban. Bivalirudin is specifically approved for use during percutaneous coronary intervention (PCI), a cardiac catheterization procedure that requires anticoagulation that is monitored with a rapid, bedside whole blood clotting time test called the activated clotting time (ACT). If a patient has PCI with bivalirudin, the ACT is used.

Only warfarin, rivaroxaban, dabigatran and apixaban are administered orally. The other anticoagulants are administered parenterally.

See Table 11–22 for a summary of the mechanisms of action and the laboratory tests to measure the anticoagulation effect of these various anticoagulant therapies.

Antiplatelet Therapies

Patients with arterial thrombosis, in particular myocardial infarction or ischemic stroke, are usually treated with antiplatelet therapy, to inhibit platelet activation. Unlike many anticoagulant therapies, laboratory testing is not yet routinely used to monitor antiplatelet therapy. Studies are ongoing to determine if laboratory monitoring of platelet therapy is beneficial. A variety of platelet-inhibiting medications are available. Aspirin irreversibly inhibits cyclooxygenase in platelets, which prevents the generation of thromboxane A2 from arachidonic acid. This inhibits platelet aggregation because thromboxane A2 triggers the release of platelet granules and activates other platelets. Several GP IIb/IIIa inhibitors are available (abciximab, eptifibatide, or tirofiban). These agents inhibit platelet aggregation because GP IIb/IIIa on the platelet surface mediates platelet aggregation. Clopidogrel, prasugrel, and ticagrelor all inhibit an ADP receptor on the platelet surface, which ultimately inhibits activation of platelet GP IIb/IIIa. Dipyridamole inhibits platelets through multiple mechanisms.

A portion of this chapter, primarily the material in the section “Introduction to Hemostasis,” was adapted with permission with modifications from Laposata M, Connor AM, Hicks, DG, Phillips DK. The Clinical Hemostasis Handbook. Chicago: Year Book Medical Publishers; 1989.