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Essential thrombocythemia is a clonal stem cell disorder characterized by an overproduction of platelets, and is most commonly associated with mutations in one of three genes, JAK2, CALR, or C-MPL, although a fraction of patients termed “triple negative” carry wild-type alleles at these three loci*. Complications include thrombosis, hemorrhage, and progression to myelofibrosis or acute myeloid leukemia. Diagnosis requires exclusion of reactive thrombocytosis and other myeloid malignancies associated with an elevated platelet count. Therapy, when appropriate, is aimed at reducing thrombotic complications and includes modification of known cardiovascular risk factors and antiplatelet therapy for the majority of such patients. Those at high risk of thrombosis are also considered for cytoreductive therapy with agents such as hydroxyurea, anagrelide, or interferon-α. Although the majority of patients can expect to live for many years, mortality rates are increased compared with the general population as a consequence of disease complications.

Acronyms and Abbreviations

AML, acute myeloid leukemia; CALR, calreticulin gene; CML, chronic myeloid leukemia; ET, essential thrombocythemia; JAK2, Janus family of tyrosine kinases type 2; C-MPL, the thrombopoietin receptor; MPN, myeloproliferative neoplasm; PCR, polymerase chain reaction; PMF, primary myelofibrosis; PV, polycythemia vera; RARS-T, refractory anemia with ringed sideroblasts and thrombocytosis; STAT, signal transducer and activator of transcription.

*This chapter was written by Philip A. Beer and Anthony R. Green in the ninth edition and portions of that text have been retained.


Essential thrombocythemia (ET), is one of the eight neoplasms, classified as a myeloproliferative disease by the World Health Organization and more conventionally as one of the four classical myeloproliferative neoplasms (MPNs) including polycythemia vera (PV) and primary myelofibrosis (PMF). ET is a clonal hematopoietic stem cell disorder characterized by thrombocytosis and associated with thrombotic and hemorrhagic complications. First recognized as a specific disease entity in 19341 and as a clonal disorder in 1981,2 ET shares clinical and pathologic similarities with other MPNs, particularly PV and PMF.


The annual incidence of ET is approximately 2 per 100,000 population and appears slightly more common in females.3,4 Patients may present at any age, although ET is largely a disorder of later life, with a peak incidence between the ages of 50 and 70 years. Presentation in childhood is rare but well recognized.


Although the genetics of the disorder is increasingly understood, little is known about the precise etiology of this disorder, although environmental factors such as exposure to radiation have been implicated in the genesis of other MPNs.5 Both registry data and kindred studies suggest a familial tendency to develop MPNs, including ET.6,7 This predisposition appears to be explained in part by inheritance of a specific haplotype that contains the Janus family of tyrosine kinases type 2 ...

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