Erythropoietin measurements in the differential diagnosis of polycythaemia.

An in vitro bioassay has been used to measure erythropoietin levels in extracts of serum from 74 patients with polycythaemia. Patients with polycythaemia vera in relapse had subnormal or undetectable levels which did not increase after challenge by venesection. In polycythaemia secondary to hypoxaemia serum erythropoietin concentrations were generally elevated with a further elevation occurring after venesection. Where polycythaemia was secondary to renal polycystic disease or tumours increased levels of erythropoietin could not be consistently demonstrated. In these patients the serum erythropoietin was unaffected by venesection. In other patients in whom the cause for polycythaemia was unknown examples of each of the above patterns were seen. Erythropoietin investigations in such patients may help in the elucidation of the underlying pathology.