Correlation between bioassay and radioimmunoassay for erythropoietin in human serum and urine concentrates

Both immunoreactive erythropoietin (Ep) and biologically active Ep were measured in 23 samples of human serum and 21 concentrates of human urine. Immunoreactive Ep was measured by radioimmunoassay (RIA). Biological activity was determined in the plethoric mouse bioassay in which 59Fe incorporation was converted to units of Ep from standard reference curves. Low values for Ep were determined from standard curves plotted as probits to improve sensitivity for levels of Ep as low as 30 mU/ml. Ep levels in 35 samples ranged between 30 and 1000 mU/ml by both assays; in 9 samples Ep was 15.2-37.5 mU/ml by RIA but was not detectable by bioassay. Analysis of the data for the 35 samples in which Ep could be measured by both assays showed a strong correlation between the values obtained by the two assays. These results indicate that the RIA used in these experiments detects biologically active Ep in human serum and urine when it is present in amounts only moderately higher than normal. The ultrafiltration method used for preparation of urine samples was effective in concentrating Ep in some urines, but the results were too erratic and nonquantitative to permit its use as a method for quantifying human urinary Ep excretion