Chronic myeloid neoplasms
Myeloproliferative neoplasms (MPN)
Essential thrombocythemia

Topic Completed: 1 August 2011

Revised: 12 March 2019

Copyright: 2002-2019,, Inc.

PubMed Search: Essential thrombocythemia [title]

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Cite this page: Sangle N. Essential thrombocythemia. website. Accessed March 22nd, 2019.
Diagrams / tables

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Drawing of JAK2 mutation

Drawing of c-Mpl mutation

Proposed criteria for ET

WHO 2008 diagnostic algorithm

Clinical features
  • See MDS/MDN - unclassifiable for refractory anemia with ringed sideroblasts plus essential thrombocythemia
  • Prevalence of 2 - 3 per million (Am J Hematol 2008;83:359)
  • Median age of 50 - 60 years at diagnosis but may occur at any age; 2/3 female
  • Rarely occurs in children (must rule out hereditary thrombocytosis)
  • Closely related to polycythemia vera but no increased red blood cell mass
  • More than half of the cases are detected due to routine peripheral blood count
  • Increased proliferation usually confined to megakaryocytes with platelet count > 450 × 109/ L
  • Thrombosis and hemorrhage common, due to qualitative and quantitative abnormalities in platelets
  • Indolent, long asymptomatic periods alternating with thrombotic or hemorrhagic crises; may rarely progress to myelofibrosis
  • Median survival is 10 - 15 years, hence life expectancy is almost normal (Orphanet J Rare Dis 2007;2:3)
  • Bone marrow examination necessary to exclude other myeloproliferative neoplasms (Arch Pathol Lab Med 1991;115:475)

Diagnostic criteria (WHO 2008): must meet all 4 major criteria
  • Criterion #1: sustained platelet count of 450 x 109 / L or more
  • Criterion #2: megakaryocyte proliferation with large and mature morphology, no or little granulocyte or erythroid proliferation
  • Criterion #3: does not meet WHO criteria for CML, PV, PMF, MDS or other myeloid neoplasm
  • Criterion #4: demonstration of JAK2 V617F or other clonal marker OR no evidence of reactive thrombocytosis

Diagnostic criteria (WHO 2001):
  • Not currently used: sustained platelet count > 600K, marrow shows proliferation mainly of megakaryocytes with enlarged mature forms, no other causes of thrombocytosis (thus, is a diagnosis of exclusion), Table
  • High risk patients (for thrombosis) get cytoreductive therapy (hydroxyurea) and low dose aspirin
Microscopic (histologic) description
  • Peripheral blood: abnormally large platelets and increased platelets; bizarre shapes, pseudopods and agranular forms
  • Bone marrow: mildly hypercellular, increased number and large forms of megakaryocytes with deeply lobulated or hyperlobulated nuclei (staghorn nuclear appearance), megakaryocytes dispersed throughout the marrow with occasional loose clusters; delicate reticulin fibers (myelofibrosis absent / minimal) but no overt fibrosis
  • Increased angiogenesis (CD34+)
  • Usually no dyserythropoiesis, dysgranulopoiesis, macrocytosis or monocytosis
  • No increased trilineage proliferation; no highly bizarre / dysplastic megakaryocytes, no predominance of small megakaryocytes with monolobulated nuclei

Diagnostic criteria for post ET myelofibrosis:
  • Required criteria: established diagnosis of ET per WHO criteria; and marrow fibrosis grade 2 - 3 (on 0 - 3 scale)
  • Additional criteria (2 required): anemia or decrease in Hb by 2 gm/dL from the baseline level; leukoerythroblastic peripheral blood; splenomegaly; rise in LDH levels; constituitional symptoms including fever, night sweats, > 10% weight loss in 6 months
Microscopic (histologic) images

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Peripheral blood:

Clusters of large platelets (platelet count is 1700K)

Increased megakaryocytes (many megakaryocytes are unusually large) in mildly hypercellular marrow of 66 year old woman with platelet count of 1200K

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Bone marrow smear:

Increased megakaryocytes

Molecular / cytogenetics description
  • Adults - JAK2 V617F mutation in up to 75% (Blood 2006;108:1865)
  • JAK2 mutant allele burden contributes to clinical phenotype (Haematologica 2008;93:41)
  • Also W515L and W515K mutations in thrombopoietin receptor c-Mpl (J Transl Med 2006;4:41)
  • May have del(20q) and unbalanced translocations between 1q and 7p
  • Must rule out BCR-ABL1 fusion gene status
  • Children - usually have no JAK2 mutation
Differential diagnosis
  • Other myeloproliferative neoplasms
  • An increase in erythroid / granulocytic population should raise suspicion of prodromal stage polycythemia vera (Acta Haematol 2005;113:213)
  • Granulocytic proliferation with bizarre or dysplastic megakaryocytes is suggesive of prefibrotic stage of PMF
  • Reactive thrombocythemia (inflammatory disorders, asplenism, infection, connective tissue disorders, metastatic cancer, lymphoproliferative disorders and iron deficiency: no JAK2 or c-Mpl mutations)
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