A 75-year-old man was investigated for persistent thrombocytosis/leukocytosis. He had history of night sweats and chronic anemia, but no thrombotic events. He had no splenomegaly and lactate dehydrogenase was persistently elevated (380 U/L). Complete blood count showed: leukocytes, 14.58 × 10⁹/L; hemoglobin, 113 g/L; mean corpuscular volume, 82.3 fL; and platelets, 1203 × 10⁹/L. Peripheral blood smear showed a leukoerythroblastic reaction with marked thrombocytosis (panel A; original magnification ×60, Wright's stain). Marrow aspirate was hypercellular with 1% blasts, megakaryocytic proliferation and atypia including dysmegakaryopoiesis (panel B inset; original magnification ×100 oil immersion, Wright-Giemsa stain), granulocytic hyperplasia, and megaloblastic erythropoiesis with dysplasia (panel B; original magnification ×100 oil immersion, Wright-Giemsa stain). The iron stain showed 40% ring sideroblasts (panel C; original magnification ×100 oil immersion, Perls' stain). Biopsy was consistent with prefibrotic/early primary myelofibrosis (prePMF) with increased abnormal megakaryocytes (panel D; original magnification ×40, hematoxylin and eosin stain) and MF-1 reticulin fibrosis. JAK2-V617F mutation was present. Cytogenetics was normal. A diagnosis of myelodysplastic/myeloproliferative neoplasms with ring sideroblasts and thrombocytosis (MDS/MPN-RS-T) was rendered per 2016 World Health Organization revision, although the patient qualified for 2 major and 3 minor criteria for prePMF.

MDS/MPN-RS-T was previously known as refractory anemia with ring sideroblasts associated with marked thrombocytosis. About 90% of patients have a SF3B1 mutation (not tested in this case). It can have both clinical/pathological features of BCR-ABL1⁻ MPN including prePMF and requires vigilant searching for dysplasia and ring sideroblasts.