Diagnosing polycythemia vera: a paradigm shift.

The primary objective during the evaluation of erythrocytosis is to ascertain the presence or absence of polycythemia vera (PV). Because of prognostic and treatment differences, PV must be distinguished from relative polycythemia and secondary erythrocytosis. This distinction is currently accomplished through the laboratory measurement of red blood cell mass, plasma volume, and arterial oxygen saturation and determination of oxygen pressure at 50% hemoglobin saturation (P50). Furthermore, according to the Polycythemia Vera Study Group guidelines, the demonstration of an increased red blood cell mass is an absolute criterion for the diagnosis of PV. This article discusses the use of the serum erythropoietin level and endogenous erythroid colony assay as a potential alternative in the diagnosis of PV.