Table 1.

Summary of WHO Criteria for MPN

Polycythemia Vera	Essential Thrombocythemia	Prefibrotic Myelofibrosis	Primary Myelofibrosis
Major criteria
HGB > 16.5 g/dL (men), >16 g/dL (women) or HCT > 49% (man) and >48% (women) or increased red cell mass. BM biopsy showing hypercellularity for age with trilineage growth, including prominent erythroid, granulocytic and megakaryocytic proliferation with pleomorphic, mature megakaryocytes. Presence of JAK2V617F or JAK2 exon 12 mutation.	Platelet count ≥450 x 109/L BM biopsy showing proliferation mainly of the megakaryocytic lineage with increased numbers of enlarged, mature megakaryocytes with hyperlobulated nuclei. No significant increase or left shift in granulopoiesis or erythropoiesis and very rarely minor (grade 1) increase in reticulin fibers. Not meeting WHO criteria for other myeloid neoplasms Presence of JAK2, CALR, or MPL mutation.	Megakaryocytic proliferation and atypia, without reticulin fibrosis > grade 1, accompanied by increased age-adjusted BM cellularity, granulocytic proliferation, and often decreased erythropoiesis. Not meeting WHO criteria for other myeloid neoplasms. Presence of JAK2, CALR, or MPL mutation or presence of another clonal marker, or absence of minor reactive BM reticulin fibrosisb	Presence of megakaryocytic proliferation and atypia accompanied by either reticulin and/or collagen fibrosis grades 2 or 3. Not meeting WHO criteria for other myeloid neoplasms. Presence of JAK2, CALR, or MPL mutation or presence of another clonal marker, or absence of reactive BM fibrosisb
Minor criterion		Minor criteria (confirmed in two consecutive determinations)
Subnormal serum EPO level.	Presence of a clonal marker or absence of evidence for reactive thrombocytosis.	Anemia not attributed to a comorbid condition Leucocytosis ≥11 x 109/L Palpable splenomegaly LDH increased	Anemia not attributed to a comorbid condition Leukocytosis ≥11 x 109/L Palpable splenomegaly LDH increased Leukoerythroblastosis
Patients must meet all three major criteria or the first two major criteria and the minor criteriona	Patients must meet all four major criteria or the first three major criteria and the minor criterion	Patients must meet all 3 major criteria, and at least 1 minor criterion.	Patients must meet all 3 major criteria, and at least 1 minor criterion.

Notes: ^aCriterion number 2 (BM biopsy) may not be required in cases with hemoglobin levels > 18.5 g/dL in men (hematocrit > 55.5%) or > 16.5 g/dL in women (hematocrit > 49.5%), if major criterion 3 and the minor criterion are present. ^bFibrosis secondary to infection, autoimmune disorder or other chronic inflammatory conditions, lymphoid neoplasm, metastatic malignancy, or toxic myelopathies. Data from Arber et al.³

Abbreviations: WHO, World Health Organization; HGB, hemoglobin; HCT, hematocrit; BM, bone marrow; EPO, erythropoietin; LDH, serum lactate dehydrogenase.