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  • The upper limit of a normal platelet count is usually between 350 × 109/L and 450 × 109/L depending on the clinical laboratory and specific method used.

  • Table 42–1 presents the major causes of elevation of the platelet count above the normal limit.




  • Essential thrombocythemia (ET) is a clonal disorder of multipotential hematopoietic progenitor cell/stem cell and is a chronic myeloproliferative neoplasm related to polycythemia vera and primary myelofibrosis.

  • Approximately 50% of patients express a mutant form of the Janus (JAK)2 signaling kinase (JAK2 V617F) found in several myeloproliferative neoplasms (polycythemia vera, primary myelofibrosis, rare cases of myelodysplastic syndromes). The mutant allele is almost invariantly found in one copy per cell in patients with essential thrombocythemia, and leads in vivo to hematopoietic growth factor hypersensitivity, a hallmark of the disease. A very small fraction of patients display other mutations of JAK2.

  • Mutations in calreticulin (~35% of patients) or of the thrombopoietin receptor, Mpl gene (~5% of patients) result in approximately 85% to 90% of the marrow and blood cells of patients with ET having one of these three gene mutations.

  • Patients who do not express a mutant form of JAK2 usually display lower hemoglobin concentrations than patients with JAK2 V617F.

Clinical Features

  • The criteria used for the diagnosis of ET are shown in Table 42–2.

  • ET usually develops between ages 50 and 70. Sex distribution is slightly skewed toward women, especially in younger patients.

  • Because platelet counts are now often done routinely, the disorder is being discovered in younger individuals and in patients who are asymptomatic.

  • Rare familial cases have been reported.

  • Constitutional or hypermetabolic symptoms are very uncommon.

  • Mild splenomegaly is found in 40% to 50% of patients.

  • Patients may have ecchymoses and bruising due to functional platelet deficiencies, or due to acquired von Willebrand disease if platelet counts are very high.

  • Bleeding and thrombotic complications are major causes of morbidity and mortality. Table 42–3 summarizes the risks of thrombosis or bleeding.

  • Bleeding is common and is characteristic of platelet or vascular disorders: mucosal, gastrointestinal, cutaneous, genitourinary, and postoperative.

  • Use of aspirin may occasionally ...

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