Aldosterone secretion rate: radioimmunoassay versus double isotope dilution derivative assay

A simple and reliable radioimmunoassay for the measurement of the aldosterone secretion rate has been developed. In this method the urinary pH 1-hydrolyzed aldosterone is acetylated to its 21-monoacetate and purified by a single paper chromatogram. The isolated aldosterone 21-monoacetate is radioimmunoassayed using the N.I.H. antiserum lot 088. From the value obtained and the tritium amount present in the eluate the specific activity is calculated. The following results disclosed the reliability of the method: 1. The values obtained were in perfect agreement with values obtained by a double isotope assay. 2. The aldosterone secretion rate values calculated did not change upon extension of chromatographic steps. 3. Immunochromatographic analysis showed a sharp peak in the aldosterone 21-monoacetate area without significant background immunoreactivity. 4. The steroid concentration measured decreased linearly with dilution of the paper eluate. 5. Identical values were obtained with three antisera, raised against different aldosterone immunogens. In our laboratory no reliable results could be obtained when the conversion to the aldosterone 21-monoacetate was omitted and the specific activity was calculated after radioimmunoassay of chromatographically purified, pH 1-hydrolyzed, aldosterone per se. In that case the values obtained were systematically lower than the double isotope values, a problem which is discussed.