Radioimmunoassay of aldosterone.

Two radioimmunoassays for the urinary acid labile conjugate of aldosterone have been developed. The simpler assay (urinary immuno-reactive aldosterone) measures the acid labile conjugate of aldosterone plus some other unidentified compound which is immunologically similar to aldosterone. The second method (urinary aldosterone) is highly specific for the acid labile conjugate of aldosterone. Antisera against aldosterone were prepared by injecting rabbits with a steroid-protein conjugate. The conjugate was prepared by coupling aldosterone 21-hemisuccinate to bovine serum albumin or keyhole limpet hemocyanin by use of a water soluble carbodiimide. Antisera were obtained that had titers as high as 1 to 2,000 and were highly specific for aldosterone. The standard curve obtained had a range from 50 to 600 pg, which is sensitive enough to measure less than 1 um-g of aldosterone in a 24 hour urine specimen. Two methods for separation of antibody found from free aldosterone were evaluated. A procedure based upon precipitation of the aldosterone antibody by use of goat anti-rabbit gamma-globulin was shown to be superior to adsorption of free aldosterone by dextran-coated florisil. The former procedure gave higher specificity and precision and lower blank values. The urinary immuno-reactive aldosterone assay and the urinary aldosterone assay have been used to determine the effect of sodium balance on aldosterone excretion. Both assays showed that aldosterone excretion was inversely related to sodium excretion when urinary sodium was less than 200 mEq/day. Determination of intra-assay and inter-assay precision of the immuno-reactive aldosterone assay gave coefficients of variation of 3.5 percent and 9.5 percent respectively. Recovery of aldosterone added to urine was 93 ± 7.6 percent at 6 um-g/24 hours and 24 um-g/24 hours respectively. Urinary immuno-reactive aldosterone levels were determined in several physiological and pathological states. In four totally adrenalectomized patients the average urinary immuno-reactive aldosterone level was 1.3 um-g/day (range 0.3 to 2.9 um-g/day). The average value obtained in the third trimester of pregnancy was 1.25.0 um-g/day (82 to 186 um-g/day were obtained in patients with secondary aldosteronism. When urinary immuno-reactive aldosterone levels were determined in 26 patients with essential hypertension, 19 patients had urinary excretion levels within normal limits. The seven patients with elevated excretion rates included three patients who had accelerated hypertension and one who had liver disease. This urinary immuno-reactive aldosterone assay has been utilized in the diagnosis of six cases of primary aldosteronism. All six patients had elevated levels of immuno-reactive aldosterone when urinary sodium was greater than 200 mEq/day. However, when urinary sodium was less than 200 mEq/day, four of the six patients had urinary immuno-reactive aldosterone levels within the normal range.