Idiopathic erythrocytosis and other non-clonal polycythemias.

Idiopathic erythrocytosis (IE) is characterized by an increase of red blood cell mass without an identified cause. Its diagnosis is based on the exclusion of polycythemia vera (PV), secondary acquired polycythemias and various congenital primary and secondary polycythemias. The frequency of IE has been estimated to be 1.1 per 1000 subjects, which is higher than that observed in PV. Heterogeneous mechanisms underlying IE have been suggested, including 'early' PV and unrecognized secondary or congenital polycythemia. However, the transition of a patient initially classified as IE into PV is a rare occurrence, when more sophisticated diagnostic techniques are employed. IE is a stable disease with a low thrombotic risk and a low, if any, tendency to spontaneous progression to acute leukemia or myelofibrosis. Phlebotomy in patients with IE is controversial. Myelosuppressive drugs should be avoided since their use is associated with evolution into acute leukemia in about 10% of patients.