Unifying Concepts

Diagnostic Criteria for Essential Thrombocytosis (ET)

Essential Thrombocytosis (ET) is the only chronic myeloproliferative disorder (MPD) without a specific phenotype. Because isolated thrombocytosis can be the initial clinical manifestation of polycythemia vera (PV), primary myelofibrosis (PMF), or chronic myelogenous leukemia, ET is not only a diagnosis of exclusion, it should also not be considered a single disease entity.

Diagnostic criteria for essential thrombocytosis (ET)

  • Persistent thrombocytosis more than 400000/µL in the absence of a reactive cause*
  • Absence of iron deficiency (normal serum ferritin for sex)
  • JAK2 V617F assay (peripheral blood; expression establishes the presence of an MPD but not its type; absence does not exclude an MPD)
  • Hemoglobin less than 16 g/dL in a man or less than 14 g/dL in a woman (hematocrit < 47% in a man or < 44% in a woman) in the absence of splenomegaly; otherwise, red cell mass and plasma volume determinations are mandatory if a JAK2 V617F assay is positive
  • Negative Bcr-Abl FISH (peripheral blood) if a JAK2 V617F assay is negative
  • If there is anemia, macrocytosis, or leukopenia, or evidence of extramedullary hematopoiesis (ie, circulating nucleated erythrocytes, immature myelocytes, or splenomegaly), a bone marrow examination (including flow cytometry and cytogenetics) is mandatory regardless of JAK2 V617F expression status

MPD indicates myeloproliferative disorder; and FISH, fluorescent in situ hybridization.
*As indicated in the text, MPD patients represent only a minority of thrombocytosis patients in general but constitute most of those with persistent thrombocytosis in the absence of a definable cause.


WHO Diagnostic Criteria for Essential Thrombocythemia

Diagnosis requires all major criteria or the first three major criteria plus a minor criterion.

Major criteria

  • Platelet count ≥450,000 per cubic millimeter
  • Bone marrow biopsy showing proliferation mainly of the megakaryocytic lineage, with increased numbers of enlarged, mature megakaryocytes with hyperlobulated nuclei; no substantial increase or left shift in neutrophil granulopoiesis or erythropoiesis; in rare instances, minor (grade 1) increase in reticulin fibers
  • Criteria for BCR-ABL1+ positive chronic myeloid leukemia, polycythemia vera, primary myelofibrosis, or other myeloid neoplasm not met
  • JAK2 V617F, CALR, or MPL mutation

Minor criteria
Presence of clonal marker or of evidence of reactive thrombocytosis



  1. Spivak JL, Silver RT. The revised World Health Organization diagnostic criteria for polycythemia vera, essential thrombocytosis, and primary myelofibrosis: an alternative proposal. Blood. 2008 Jul 15;112(2):231-9. [Medline]
  2. Vannucchi AM, Guglielmelli P, Tefferi A. Polycythemia vera and essential thrombocythemia: algorithmic approach. Curr Opin Hematol. 2018 Mar;25(2):112-119. [Medline]
  3. Tefferi A, Pardanani A. Essential Thrombocythemia. N Engl J Med. 2019;381(22):2135-2144. [Medline]


Created Jan 21, 2019.

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