Low serum erythropoietin--a strong diagnostic criterion of primary polycythaemia even at normal haemoglobin levels.

Estimations of serum erythropoietin level by enzyme immunoassay (EIA) kit were made in 42 patients with primary polycythaemia, and in a comparison group consisting of 41 patients with secondary polycythaemia and 47 patients with idiopathic erythrocytosis. The majority of patients with primary polycythaemia were undergoing treatment by venesection and therefore had Hb levels in the normal range at the time of erythropoietin estimation. In primary polycythaemia, 64% of the first samples taken from each patient were below the reference range for serum erythropoietin in normal individuals. When two samples were taken from each patient 72% had low values in one or both samples. In the comparison group, analysis of those patients who had two samples taken showed only one individual with secondary polycythaemia and none with idiopathic erythrocytosis who had a serum erythropoietin level below the reference range. The finding of low serum erythropoietin in patients with primary polycythaemia, even when Hb levels are normal due to venesection, is of high diagnostic specificity (few false positive results) and useful diagnostic sensitivity (28% false negative results). It is proposed that future diagnostic criteria of primary polycythaemia should include the finding of a serum erythropoietin level below the lower limit of normal in at least one of two serum samples taken on different occasions.