biostudies-literatureGordeuk VRNHLBI NIH HHS653-658https://www.ebi.ac.uk/biostudies/studies/S-EPMC6442963biostudies-literatureUnknown104(4)Here we critically evaluate the role of elevated hematocrit as the principal determinant of thrombotic risk in polycythemia and erythrocytosis, defined by an expansion of red cell mass. Since red cell volume determination is no longer readily available, in clinical practice, polycythemia and erythrocytosis are defined by elevated hemoglobin and hematocrit. Thrombosis is common in Chuvash erythrocytosis and polycythemia vera. Although the increased thrombotic risk is assumed to be due to the elevated hematocrit and an associated increase in blood viscosity, thrombosis does not accompany most types of erythrocytosis. We review studies indicating that the occurrence of thrombosis in Chuvash erythrocytosis is independent of hematocrit, that the thrombotic risk is paradoxically increased by phlHaematologicaRe-evaluation of hematocrit as a determinant of thrombotic risk in erythrocytosis.PMC6442963R01 HL137991Gordeuk VRKey NSPrchal JTfalseRe-evaluation of hematocrit as a determinant of thrombotic risk in erythrocytosis.Here we critically evaluate the role of elevated hematocrit as the principal determinant of thrombotic risk in polycythemia and erythrocytosis, defined by an expansion of red cell mass. Since red cell volume determination is no longer readily available, in clinical practice, polycythemia and erythrocytosis are defined by elevated hemoglobin and hematocrit. Thrombosis is common in Chuvash erythrocytosis and polycythemia vera. Although the increased thrombotic risk is assumed to be due to the elevated hematocrit and an associated increase in blood viscosity, thrombosis does not accompany most types of erythrocytosis. We review studies indicating that the occurrence of thrombosis in Chuvash erythrocytosis is independent of hematocrit, that the thrombotic risk is paradoxically increased by phl2019-01-01T00:00:00Z2019 Apr2025-04-20T01:29:00.924Z2019-07-30T07:01:40ZS-EPMC64429633087237010.3324/haematol.2018.210732