Highly sensitive and rapid radioimmunoassay for aldosterone in plasma and cell culture medium

A radioimmunoassay for aldosterone was developed using a sensitive and specific antibody and 125Iodinated-aldosterone. This assay could be used for direct determination of aldosterone in cell culture medium or after extraction of aldosterone from plasma by solid phase procedure using C18 Sep-pak cartridges. The very low cross-reactivity of the antibody with cortisol and corticosterone (0.005% and 0.04% respectively) would allow the direct determination of aldosterone in cell culture medium without any prior extraction step. Since the incubation is performed at room temperature for 1 h and then, at 4°C for 15 min, the results can be obtained in less than 3 h. The assay was linear from 0.5 fmol/tube to 1500 fmol/tube with an ED 50 at 30 fmol/tube. The accuracy of the assay estimated using spiked plasma samples with a known amount of aldosterone give a coefficient of correlation of 0.97 ( n = 10) between the aldosterone concentrations found and expected levels. The within-assay variability for plasma aldosterone varied from 4.7 to 11.1% and the between-assay variability ranged from 13.9 to 14.2%. The coefficient of correlation between plasma aldosterone measured by this new assay or by a current assay was 0.8 ( n = 43). In summary, the combination of a shortened incubation time with a simple solid phase extraction for aldosterone in serum samples represents the major advantage of the present assay over the current methodology which usually requires a chromatographic separation of the mineralocorticoid prior to radioimmunoassay. Therefore this assay would be useful in experimental studies as well as in clinical laboratory.