Serum prolactin in uraemia: correlations between bioactivity and activity in two immunoassays

The clinical significance of hyperprolactinaemia in uraemic patients is uncertain and discrepancies between immunoactivity and biological activity of serum hPRL have been reported. We have modified the Nb2 cell bioassay to improve specificity for hPRL and used this assay to measure hPRL bioactivity in sera from 26 uraemic patients and 40 control subjects. Seventeen patients were receiving regular haemodialysis and 9 continuous ambulatory peritoneal dialysis. Levels of hPRL bioactivity were compared with hPRL immunoactivity measured by RIA (PRL-RIA) and by immunoradiometric assay (PRL-IRMA). Serum hPRL levels measured by all three assays were significantly elevated in uraemic patients compared with control subjects (P less than 0.001). The immunoradiometric method gave significantly lower results than RIA in control subjects but not in uraemic patients (P less than 0.05). There was no significant difference in mean ratio of hPRL bioactivity to PRL-RIA between patients and control subjects (1.18 +/- 0.05 vs 1.11 +/- 0.03, mean +/- SEM). The ratio of hPRL bioactivity to PRL-IRMA was slightly decreased in uraemic patients compared with controls (P = 0.05). Serum hPRL bioactivity was closely correlated with immunoactivity in both immunoassays (r greater than or equal to 0.96) in patients and controls. These results confirm that elevated serum hPRL levels in uraemic patients represent biologically active hormone which may contribute to hypogonadism.