A direct radioimmunoassay for aldosterone in unextracted serum and plasma

We describe a novel radioimmunoassay procedure for the direct estimation of aldosterone in unextracted plasma and serum samples, in which interfering binding proteins are digested by Proteinase K (Tritirachium alkaline proteinase, EC 3.4.21.14), a powerful proteolytic enzyme. Heating at 75 degrees C for 15 min inactivates the enzyme before radioimmunoassay. Alternatively, EDTA may be used to inactivate the enzyme. The antibody-bound fraction is then precipitated with polyethylene glycol and isolated by centrifugation. This easy method eliminates extraction and purification and gives accurate and reliable results. The total time required for 100 estimations, including counting time, is about 6 h.