Table 1.

World Health Organization 2008 diagnostic criteria for myeloproliferative neoplasm

Criterion	Polycythemia vera	Primary myelofibrosis	Essential thrombocythemia
Major	Hemoglobin > 18.5 g/dL (men), > 16.5 g/dL (women) ora Presence of JAK2 V617F or JAK2 exon 12 mutation	Megakaryocyte proliferation and atypiab, accompanied by either reticulin and/or collagen fibrosis, orc Not meeting WHO criteria for PV, CML, MDS, or other myeloid neoplasm Demonstration of JAK2 V617F or other clonal marker or no evidence of bone marrow fibrosis	Sustained platelet count > 450 × 109/L Proliferation of megakaryocytes with enlarged, mature morphology Not meeting WHO criteria for PV, PMF, CML, MDS or other myeloid neoplasm Demonstration of JAK2 V617F or other clonal marker, or no evidence for reactive thrombocytosis
Minor	BM hypercellularity with trilineage myeloproliferation Subnormal serum erythropoietin level Endogenous erythroid colony formation in vitro	Leukoerythroblastosis Increase in serum LDH level Anemia Splenomegaly

PV diagnosis requires meeting either both major criteria and one minor criterion or the first major criterion and two minor criteria. Essential thrombocythemia diagnosis requires meeting all four major criteria. PMF diagnosis requires meeting all three major criteria and two minor criteria.

JAK2, Janus kinase 2; WHO, World Health Organization; PV, polycythemia vera; CML, chronic myelogenous leukemia; MDS, myelodysplastic syndromes; PMF, primary myelofibrosis; BM, bone marrow; LDH, lactate dehydrogenase.

^aOr Hb or Hct > 99th percentile of reference range for age, sex, or altitude of residence or Hb > 17 g/dL in men, 15 g/dL in women if associated with a sustained increase of ≥ 2 g/dL from baseline that cannot be attributed to correction of iron deficiency, or elevated red cell mass > 25% above mean normal predicted value.

^bSmall to large megakaryocytes with an aberrant nuclear/cytoplasmic ratio and hyperchromatic, bulbous or irregularly folded nuclei and dense clustering.

^cOr in the absence of reticulin fibrosis, the megakaryocyte changes must be accompanied by increased marrow cellularity, granulocytic proliferation, and often decreased erythropoiesis (i.e., prefibrotic cellular-phase disease).