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In 10 JAK2V617F positive patients with early myeloproliferative essential thrombocythemia (ET) or polycythemia vera (PV) we sequentially used the Polycythemia Vera Study Group (PVSG), the Thrombocythemia Vera Study Group (TVSG) and the 2008 WHO criteria for the clinical diagnosis of essential thrombocythemia (ET) ad polycythemia vera (PV). Subsequently, we evaluated bone marrow features and laboratory and molecular markers including endogenous erythroid colony (EEC), serum erythropoietine (EPO) mutation for the classification of early MPD. Six symptomatic patients had a delayed diagnosis of slow onset latent (masked) ET and 4 patients presented with rapid onset PV associated with thrombocythemia. We could clearly distinguish three phenotypes of ET when the 2008 European clinical, molecular and pathological (2008 WHO-ECMP) criteria for ET and PV are applied. These include ET phenotype 1, ET phenotype type 2 with features of early polycythemia vera (PV) and normal red cell mass, and ET phenotype 3 associated with megakaryocytic granulocytic myeloproliferation (ET-MGM) in the absence of characteristic features of primary myelofibrosis (PMF). Through the presentation of a number of highly illustrative clinical cases during long-term follow-up we could demonstrate that diagnostic differentiation of three phenotypes of ET is important, because natural history clearly differ.