Reliability and validity of commercially available, direct radioimmunoassays for measurement of blood androgens and estrogens in postmenopausal women

In large-scale epidemiological studies on endogenous sex steroids and cance r risk, direct immunoassays of circulating hormone levels have the advantag e of being fast and comparatively inexpensive while requiring only small sa mple volumes. On the other hand, indirect assays after organic extraction a nd chromatographic prepurification have the advantage of reducing specific interferences and matrix effects and hence are thought to have better valid ity. We compared direct assays of testosterone (T, six different assays), D elta4-androstenedione (A, four assays), estrone (E-1, one assay), and 17 be ta -estradiol (E-2, five assays) with measurements obtained by an indirect assay in a representative subset of 20 postmenopausal women who were part o f a large prospective cohort study. Within-batch reproducibilities of the subject rankings by relative hormone levels were good (intraclass correlations >0.89) for all direct assays test ed. Between batches, reproducibilities generally were also acceptable (r > 0.80) to good (r > 0.90) in terms of Pearson's correlations. The between-ba tch reproducibility in terms of intraclass correlations was systematically lower in terms of Pearson's correlations, however, because of between-batch variations in the absolute scale of measurements. The relative validity of direct versus indirect assays in terms of the subjects' ranking by relativ e hormone levels was also high for most of the kits tested for T, A, and E- 1 (Pearson's correlations between 0.70 and 0.89) but was high for only two kits of five tested for E-2 (correlations of 0.86 and 0.84). On an absolute scale, mean measurement values were generally higher for direct assays tha n for the indirect assay and, for each hormone, varied substantially, depen ding on the kit used. Overall, the results of this study show that, with careful selection, comme rcial kits for direct radioimmunoassays of steroid hormones in postmenopaus al serum can be found that may allow a reliable estimation of relative risk s in epidemiological studies. However, standardization of the absolute scal e of assays remains problematic.