A method of assaying cortisol without extraction and in 1 microliter of plasma

A simplified method is described for the assay of cortisol in human plasma. It is based upon tryptic hydrolysis of cortisol binding proteins in the plasma followed by radioimmunoassay. Only 1 microliter of plasma is used, no extraction of the hormone is necessary and most of the procedure is amenable to automatization. The sensitivity of the method is 10 pg, the coefficient of variation is 8.7% intrassay and 11.5% in different assays. The accuracy and the specificity were also verified. Comparison of this simplified technique with a standard method (extraction by methylene chloride and radiocompetition with corticosteroid binding globulin) gave a correlation coefficient (r) of 0.964.