Idiopathic erythrocytosis: a disappearing entity.

Erythrocytosis results when there is an increased red cell mass and thus an increased hemoglobin. The causes can be divided into primary intrinsic defects of the erythroid progenitor cell and secondary defects, where factors external to the erythroid compartment are responsible. Both can then be further divided into congenital and acquired categories. Congenital causes include mutations of the erythropoietin receptor and defects of the oxygen-sensing pathway including VHL, PHD2 and HIF2A mutations. When fully investigated there remain a number of patients in whom no cause can be elucidated who are currently described as having idiopathic erythrocytosis. Investigation should start with a full history and examination. Having eliminated the common entity polycythemia vera, further direction for investigation is guided by the erythropoietin level. Clinical consequences of the various erythrocytoses are not clear, but in some groups thromboembolic events have been described in young patients. Evidence is lacking to define best management, but aspirin and venesection to a target hematocrit should be considered.