Immunoreactive erythropoietin in serum. I. Evidence for the validity of the assay method and the physiological relevance of estimates.

A radioimmunoassay for erythropoietin (Ep) in human serum is described. Evidence of assay validity includes parallelism of dilution curves of the 2nd International Reference Preparation (IRP) (used as standard) with test sera, a fall in serum immunoreactive Ep following blood transfusion and an increase in serum immunoreactive Ep with increasing severity of anaemia except in anaemias associated with severe renal failure. The geometric mean estimate of serum immunoreactive Ep in 93 normal men and women was 13.3 miu/ml above the low erythropoietin serum pool used as baseline for the assay. Extra-renal erythropoietin was detectable in the assay and gave a dilution curve parallel to the 2nd IRP.