Polycythemia Vera, the Hematocrit, and Blood-Volume Physiology

Marchioli et al.1 report in the Journal that a hematocrit target of less than 45% for therapeutic phlebotomy reduces the risk of thrombosis in patients with polycythemia vera. In the genomic era, readers may question attention given to a measurement as mundane as the hematocrit, but this study resolves a half-century of debate about the role of phlebotomy in polycythemia vera and has ramifications for diagnosis and management.Polycythemia vera is a unique myeloproliferative disorder in which there is overproduction of morphologically normal erythrocytes, granulocytes, and platelets, a phenotype that is caused by a mutation (V617F) in JAK2, encoding a . . .