Serum erythropoietin in the diagnosis of polycythaemia and after phlebotomy treatment.

Serum erythropoietin (S-Epo) was measured with a radiommunoassay method in 36 patients with polycythaemia vera, 17 patients with secondary polycythaemia and 14 patients with relative polycythaemia. The diagnoses were made without the aid of the S-Epo values. It was found that S-Epo was below the reference range in 34/36 patients with polycythaemia vera (mean 2.1 +/- 1.0 U/l), elevated in all cases of secondary polycythaemia (mean 121.7 +/- 242 U/l) and normal in all but one of the cases with relative polycythaemia (mean 7.0 +/- 2.5 U/l). Previous studies of S-Epo levels in the differential diagnosis of polycythaemia have shown significant differences between the means of the groups but a considerable overlap. After phlebotomy treatment to normal haematocrit levels, the S-Epo levels remained subnormal in most of the polycythaemia vera patients even after 18 months at a haematocrit around 45%. Two patients who had been kept at normal haematocrit for 6 and 7 years both had subnormal S-Epo. We conclude that with an Epo assay method of high sensitivity and specificity it is possible to differentiate between different forms of polycythaemia with a high degree of certainty, even between patients with relative polycythaemia and polycythaemia vera patients. The reason why S-Epo remains low in spite of a normal haematocrit in treated polycythaemia vera patients is not known.