Serum erythropoietin measurements using the fetal mouse liver cell culture: the importance of reduction of variation in the specific activity of radioiron--transferrin.

Because of varying iron--transferrin concentrations in different serum samples, and varying test serum portions within the culture, variations of the radioiron--transferrin/total iron--transferrin ratio are inevitable, when serum erythropoietin (Ep) concentrations are measured using the fetal mouse liver cell assay. It could be demonstrated that radioiron uptake is directly proportional to the specific activity of radioiron--transferrin, when the latter varies over the range which is inherent to the method. Variations of the ratio of radioiron--transferrin/total iron--transferrin were reduced by preincubating radioiron with human transferrin, and by minimizing the test serum portion of the culture. With this modified in vitro bioassay, serum Ep concentrations of 59 healthy subjects were measured. Mean serum Ep concentration was 136 +/- 66 (s.d.) mU/ml.