An immunoassay for plasma cortisol based on chemiluminescence

An immunoassay procedure for the determination of cortisol in human plasma is described, which utilizes chemiluminescence as the end point. A cortisol-isoluminol conjugate serves as the chemiluminescent marker. The light emission by this conjugate upon oxidation is delayed by prior incubation with anti-cortisol IgG, but not by unrelated gamma-globulin. This delayed light emission was inhibited by cortisol in a dose-dependent manner, with a linear range of 20-1000 pg steroid/assay tube. A competitive protein binding assay based on this procedure was applied to methylene chloride extracts of cortisol from normal and pathological human plasma (2-40 micrograms/100 ml). Cortisol values obtained by this procedure agreed well with those obtained by radioimmunoassay, using the same antiserum with tritiated cortisol as the label (r = 0.98). The chemiluminescence immunoassay is comparable to radioimmunoassay with regard to sensitivity, specificity, precision and accuracy. The advantage of the new assay procedure is that it obviates the need for counting radioactivity and for separation of bound and free ligand.