Example cases in the differential diagnosis of ET. (A) Triple-negative ET in a 40-year-old female with thrombocythemia, no mutations detected. Normocellular bone marrow with a mild increase in megakaryocytes (i), occasional loose clusters (ii), and subpopulation of larger megakaryocytes with hypersegmented nuclear morphology (ii-iii) with no evidence of fibrosis (iv). Exclusion of reactive causes of thrombocytosis required to meet diagnostic criteria (Table 2).8,9,16 (B) Reactive thrombocytosis in a 24-year-old female with platelet count of 480 × 109/L, normocellular bone marrow, and no increase in megakaryocytes (i). Occasional large megakaryocytes with hypersegmented nuclei (ii) and most megakaryocytes show normal morphology (iii). CD68R (iv) shows increased macrophage activity. (C) Iron-deficient PV in a 30-year-old female with thrombocytosis, normal hemoglobin, and iron deficient. The combination of hypercellularity (i), megakaryocyte clustering and pleomorphism (ii-iii, respectively), and the WHO classification–based grade-1 fibrosis (iv) favor a diagnosis of iron-deficient PV rather than ET.8,9 (D) MDS/MPN with SF3B1 mutation and thrombocytosis in a 80-year-old female with anemia and thrombocythemia. Hypercellular bone marrow, with erythroid hyperplasia (i, iv, respectively), dyserythropoiesis (ii), and expanded megakaryopoiesis (iii). Megakaryocytes show an MPN-like morphology, and >15% ring sideroblasts on aspirate was observed; JAK2 V617F and SF3B1 mutations detected.8,9 (E) Prefibrotic primary myelofibrosis in a 70-year-old female with leucocytosis and thrombocytosis, JAK2 V617F positive. (i) Hypercellular bone marrow with expanded granulopoiesis and megakaryopoiesis. Megakaryocytes show clustering and include forms with hyperchromatic, hyperlobated nuclei (ii) as well as megakaryocytes with bulbous or cloud-like nuclear morphology (iii), with the WHO classification–based grade 1 reticulin fibrosis (iv).8,9 Hb, hemoglobin; WHO, World Health Organization.
