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.
- 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
Presence of clonal marker or of evidence of reactive thrombocytosis
- 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]
- Vannucchi AM, Guglielmelli P, Tefferi A. Polycythemia vera and essential thrombocythemia: algorithmic approach. Curr Opin Hematol. 2018 Mar;25(2):112-119. [Medline]
- Tefferi A, Pardanani A. Essential Thrombocythemia. N Engl J Med. 2019;381(22):2135-2144. [Medline]
Created Jan 21, 2019.