Determination of cortisol in human plasma by radioimmunoassay. Use of the 125I-labelled radioligand.

A radioimmunoassay for plasma cortisol featuring the gamma-emitting radioligand 125I-iodohistamine, coupled to cortisol-3-(O-carboxymethyl)-oxime, is described. The new procedure retains much of the specificity associated with the use of anti-cortisol-3-BSA sera with tritium-labelled radioligands, and has the further advantages that running costs are lower and there is a greater potential for automation. Cortisol values obtained by this procedure agree well with those obtained by a published specific radioimmunoassay using the tritiated cortisol radioligand. Specificity of the procedure was checked by comparing values obtained with and without thin-layer chromatography purication: correlation was excellent (r = 0.96). Satisfactory levels of sensitivity, precision, and accuracy were obtained.