Determination of cortisol by competitive protein binding or radiostereoassay.

A sensitive technique for plasma insulin assay based on displacement of labeled insulin from its antibody was reported by y ALLOW and BERSON in 1960 (9). Subsequently, similar methods based on competitive protein binding have been deve1oped. In 1963 MURPHY and co-workers (6) described a technique involving a dialysis using 14C-cortisol. The time required for this procedure was shortened from two days to two hours when gel filtration using Sephadex was substituted for the dialysis step (2,7) . However, the filtration columns re­ quired setting up and cooling. Recently (3), a 10-100 fold increase in sensi­ tivity was obtained by the use of tritiated steroids, and the technique was further simplified by replacing the gel filtration step with one involving the precipitation of the unbound fraction by adsorption to insoluble substances (1,4,5). Using the principIes of radiostereoassay suggested by MURPHY (6), we have developed a method for measuring cortisol instead of total corticosteroids as previously done.