Criteria
The 5th edition of the World Health Organization (WHO) classification of hematolymphoid tumors: myeloid and histiocytic/dendric neoplasms[2]
Major criteria
Platelet count ≥450,000/microliter.
Bone marrow biopsy showing proliferation mainly of the megakaryocyte lineage with increased numbers of enlarged, mature megakaryocytes with hyperlobulated nuclei. No significant increase or left shift in neutrophil granulopoiesis or erythropoiesis and very rarely minor (grade 1) increase in reticulin fibers.
Not meeting WHO criteria for BCR::ABL1+, chronic myeloid leukemia (CML), polycythemia vera (PV), primary myelofibrosis (PMF), myelodysplastic syndromes (MDS), or other myeloid neoplasms.
Presence of Janus kinase 2 (JAK2), calreticulin (CALR), or myeloproliferative leukemia virus oncogene (MPL) gene mutations.
Minor criterion
Presence of a clonal marker or absence of evidence for reactive thrombocytosis.
Diagnosis of essential thrombocythemia (ET) requires all 4 major criteria to be met or the first 3 major criteria and the minor criterion.
International consensus classification of myeloid neoplasms and acute leukemias[1]
Major criteria
Platelet count ≥450,000/microliter.
Bone marrow biopsy showing proliferation mainly of the megakaryocytic lineage, with increased numbers of enlarged, mature megakaryocytes with hyperlobulated staghorn-like nuclei, infrequently dense clusters; no significant increase or left shift in neutrophil granulopoiesis or erythropoiesis; no relevant bone marrow fibrosis (very rarely a minor increase in reticulin fibers may occur at initial diagnosis [grade 1])
Diagnostic criteria for BCR::ABL1-positive CML, PV, PMF, or other myeloid neoplasms are not met
JAK2, CALR, or MPL gene mutations.
Minor criteria
Presence of a clonal marker or absence of evidence of reactive thrombocytosis.
The diagnosis of ET requires either all major criteria or the first 3 major criteria plus the minor criteria.
British Committee for Standards in Haematology diagnostic criteria for essential thrombocythaemia (modified 2014)[41]
A1: Sustained platelet count ≥450,000/microliter.
A2: Presence of an acquired pathogenetic mutation (e.g., JAK2, CALR, or MPL genes).
A3: No other myeloid malignancy, especially the following:
PV (excluded by a normal hematocrit in an iron-replete patient)
PMF (indicated by the presence of significant bone marrow fibrosis [≥2/3 or 3/4 reticulin] and palpable splenomegaly, blood film abnormalities [circulating progenitors and tear-drop cells], or unexplained anemia)
CML (excluded by absence of BCR-ABL1 fusion gene from bone marrow or peripheral blood)
MDS (excluded by absence of dysplasia on examination of blood film and bone marrow aspirate).
A4: No reactive cause for thrombocytosis and normal iron stores.
A5: Bone marrow aspirate and trephine biopsy showing increased megakaryocyte numbers displaying a spectrum of morphology with predominant large megakaryocytes with hyperlobated nuclei and abundant cytoplasm. Reticulin is generally not increased (grades 0 to 2/4 or grade 0/3).
Diagnosis requires A1 to A3 or A1 plus A3 to A5.
Revised international prognostic score of thrombosis for essential thrombocythemia (IPSET-thrombosis)[42][43]
Very low risk
Age ≤60 years, no thrombosis history, no JAK2 V617F mutation.
Low risk
Age ≤60 years, no thrombosis history, with JAK2 V617F mutation.
Intermediate risk
Age >60 years, no thrombosis history, no JAK2 V617F mutation.
High risk
Thrombosis history at any age, or age >60 years with JAK2 V617F mutation.
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