Table 2.

Diagnostic criteria for essential thrombocythemia and post-essential thrombocythemia myelofibrosis (post-ET MF) according to the International Consensus Classification¹

	ET		Post-ET MF
Major criteria	1. Platelet count ≥ 450 × 109/L	Required criteria	1. Previous established diagnosis of ET
	2. Bone marrow biopsy showing proliferation mainly of the megakaryocytic lineage, with increased numbers of enlarged, mature megakaryocytes with hyperlobulated staghorn-like nuclei, infrequently dense clustersa; no significant increase or left shift in neutrophil granulopoiesis or erythropoiesis; no relevant BM fibrosisb		2. Bone marrow fibrosis of grade 2 or 3 (MF-2 or MF-3)
	3. Diagnostic criteria for BCR::ABL1 positive chronic myeloid leukemia, polycythemia vera, primary myelofibrosis, or other myeloid neoplasms are not met	Additional criteria	Anemia (i.e., below the reference range given age, sex, and altitude considerations) and a > 2 g/dL decrease from baseline hemoglobin concentration
	4. JAK2, CALR, or MPL mutationc		Leukoerythroblastosis
Minor criteria	1. Presence of a clonal markerd or absence of evidence of reactive thrombocytosise		Increase in palpable splenomegaly of > 5 cm from baseline or the development of a newly palpable splenomegaly
			Elevated lactate dehydrogenase level above the reference range
			Development of any 2 (or all 3) of the following constitutional symptoms: >10% weight loss in 6 months, night sweats, unexplained fever (> 37.5 °C)

The diagnosis of ET requires either all major criteria or the first 3 major criteria plus the minor criteria. The diagnosis of post-ET MF is established by the two required criteria and at least two additional criteria

^aThree or more megakaryocytes lying adjacent without other BM cells in between; in most of these rare clusters < 6 megakaryocytes may be observed, increase in huge clusters (> 6 cells) accompanied by granulocytic proliferation is a morphological hallmark of pre-PMF

^bVery rarely a minor increase in reticulin fibers may occur at initial diagnosis (MF-1)

^cIt is recommended to use highly sensitive assays for JAK2V617F (sensitivity level < 1%) and CALR and MPL (sensitivity level 1–3%)—in negative cases, consider a search for non-canonical JAK2 and MPL mutations

^dAssessed by cytogenetics or sensitive NGS techniques

^eReactive causes of thrombocytosis include a variety of underlying conditions like iron deficiency, chronic infection, chronic inflammatory disease, medication, neoplasia, or history of splenectomy