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Thrombocytopenia is one of the most frequent causes for hematologic consultation in the practice of medicine, and potentially one of the most life-threatening. Although the normal platelet count in humans (150–400 × 109/L) far exceeds the minimal level required to avoid pathologic hemorrhage (<50 × 109/L), a number of medical conditions cause either increased destruction or reduced production of platelets, increasing the risk of pathologic bleeding. This chapter discusses an approach to the diagnosis of thrombocytopenia, grouping various causes by mechanism of action, and describing our current understanding of pathogenesis, treatment, and prognostication. In the vast majority of patients, a cause for thrombocytopenia can be identified, and effective therapy instituted.

Acronyms and Abbreviations

Abbreviations and acronyms that appear in this chapter include: ADAMTS, a disintegrin and metalloproteinase with thrombospondin repeats; ALL, acute lymphocytic leukemia; APLA, antiphospholipid antibody; APS, antiphospholipid antibody syndrome; CAMT, congenital amegakaryocytic thrombocytopenia; CTP, cyclic thrombocytopenia; DIC, disseminated intravascular coagulation; EDTA, ethylenediaminetetraacetic acid; Flt, fms-like tyrosine kinase; FOG, friend of GATA1; FPS/AML, familial platelet syndrome with predisposition to acute myelogenous leukemia; GP, glycoprotein; HAART, highly active antiretroviral therapy; HELLP, hemolysis, elevated liver enzymes, and low platelet count; HIT, heparin-induced thrombocytopenia; HLA, human leukocyte antigen; HPA, human platelet antigen; HSC, hematopoietic stem cell; HUS, hemolytic uremic syndrome; Ig, immunoglobulin; IL, interleukin; ITP, immune thrombocytopenic purpura; IVIg, intravenous immunoglobulin; KMS, Kasabach-Merritt syndrome; MACA, modified antigen-capture enzyme-linked immunoadsorbent assay; MDS, myelodysplastic syndrome; NAIT, neonatal alloimmune thrombocytopenia; NICU, neonatal intensive care unit; NMMHC, nonmuscle myosin heavy chain; PAICA, platelet-associated IgG characterization assay; PAIgG, platelet-associated immunoglobulin G; RAEB, refractory anemia with excess blasts; SLE, systemic lupus erythematosus; TAR, thrombocytopenia with absent radii; TPO, thrombopoietin; TTP, thrombotic thrombocytopenic purpura; VEGF, vascular endothelial growth factor; VWF, von Willebrand factor; WAS, Wiskott-Aldrich syndrome; WASP, WAS protein.

Platelet kinetic studies have been performed to determine the pathophysiologic mechanisms in various thrombocytopenic states, particularly in complicated clinical situations. For instance, the thrombocytopenia seen in patients with HIV infection can result from many factors, including platelet destruction (because of an autoimmune mechanism or drug toxicity) or decreased platelet production because of direct megakaryocyte infection by the virus, or by marrow-based malignancy or opportunistic infection.

Platelet kinetic studies are performed using autologous platelets, which are labeled ex vivo with radioactive isotopes and then infused back into the patient. The radioisotope indium-111 (111In) oxine is most commonly used because it binds platelets very efficiently, which allows kinetic studies even in subjects with very low platelet counts.1–3 Platelet recovery, platelet survival, and platelet turnover are calculated based on the radioactivity of blood samples drawn from the patient during the several days following injection of the radiolabeled platelets. Platelet recovery is determined by the percentage of radiolabeled platelets detected in blood 1 hour after the injection. It takes approximately 10 to 12 minutes for the platelets to pass through a normal spleen; nearly one-third of reinfused platelets are ...

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