Thrombocytopenia, Heparin-Induced

Key Features

Essentials of Diagnosis

• Thrombocytopenia within 5–14 days of exposure to heparin

• Decline in baseline platelet count of 50% or greater

• Thrombosis occurs in up to 50% of cases; bleeding is uncommon

General Considerations

• Affects approximately 3% of patients exposed to unfractionated heparin and 0.6% of patients exposed to low-molecular-weight heparin (LMWH)

• Results from formation of IgG antibodies to heparin-platelet factor 4 (PF4) complexes; the antibody:heparin-PF4 complex binds to and activates platelets independent of physiologic hemostasis, which leads to thrombocytopenia and thromboses

Clinical Findings

• Patients are often asymptomatic

• Bleeding usually does not occur due to the pro-thrombotic nature of HIT

• Thrombosis (at any venous or arterial site), however, may be detected in up to 50% of patients, up to 30 days post-diagnosis

Diagnosis

• If thrombosis has not already been detected, the use of duplex Doppler ultrasound of the lower extremities should be considered to rule out subclinical deep vein thrombosis (DVT)

• A presumptive diagnosis of HIT is made when new-onset thrombocytopenia is detected in a patient (typically a hospitalized patient) within 5–14 days of exposure to heparin

• Other presentations (eg, rapid-onset HIT) are less common

• A decline of ≥ 50% or more from the baseline platelet count is typical

• The 4T score is a clinical prediction rule

  • Can be found at http://www.qxmd.com/calculate-online/hematology/hit-heparin-induced-thrombocytopenia-probability

  • Assesses pretest probability for HIT

  • However, low scores have been shown to be more predictive of excluding HIT, than are intermediate or high scores of predicting its presence

  • When HIT is clinically suspected, a screening PF4-heparin antibody enzyme-linked immunosorbent assay (ELISA) must be performed to establish HIT diagnosis

• If this PF4-heparin antibody ELISA is positive, the diagnosis must be confirmed using a functional assay (such as serotonin release assay)

• The magnitude of a positive ELISA result correlates with the clinical probability of HIT

Treatment

• Initiate as soon as the diagnosis of HIT is suspected, before results of laboratory testing is available

• Management of HIT (Table 14–5) involves the immediate discontinuation of all forms of heparin

• Despite thrombocytopenia, platelet transfusions are rarely necessary and should be avoided

• Fondaparinux is a reasonable option for clinically stable patients

• Direct thrombin inhibitor (DTI)

  • Preferred in critical illness because of the shorter duration of action

  • Argatroban should be administered immediately due to substantial frequency of thrombosis among HIT patients

  • Should be continued until the platelet count has recovered to at least 100,000/mcL, at which point treatment with a vitamin K antagonist (warfarin) may be initiated

  • Continue the DTI until therapeutic anticoagulation with warfarin has been achieved (international normalized ratio [INR] of 2.0–3.0)

  • Infusion of argatroban must be temporarily discontinued for 2 hours before the INR is obtained so that it reflects the ...