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Interference by Polar Metabolites in a Direct Radioimmunoassay for Plasma Aldosterone

The CIS Aldoctk-125 kit, a direct radioimmunoassay for plasma aldosterone, has been compared with a conventional technique involving solvent extraction. Results given by these two methods were poorly correlated (r = 0·445, n = 103), the direct assay giving higher values, particularly in patients being dialysed for renal failure. When the kit was modified to include an extraction step, results correlated well with those of the standard method (r = 0·952, n = 60). These observations suggested interference from polar metabolites. The possibility that glucuronides were responsible was investigated by measuring plasma aldosterone before and after hydrolysis with β-glucuronidase. Higher post-hydrolysis values confirmed the presence of glucuronides in plasma from normal subjects and patients with renal failure. Preliminary chromatographic studies on plasma from nine dialysis patients indicated the presence of tetrahydroaldosterone 3-glucuronide, and it is thought that this metabolite might contribute to the high values obtained with the direct assay.