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 form polar metabolites. The possibility that
glucuronides were responsible was investigated by measuring plasma
aldosterone before and after hydrolysis with
beta-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 form 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.