Development and evaluation of a simple, direct, solid-phase radioimmunoassay of serum cortisol from readily available reagents.

A simple, rapid solid-phase radioimmunoassay for serum cortisol was developed using cortisol antibody distributed by the Scottish Antibody Production Unit and commercially available radioiodinated cortisol ligand. The assay involves a 1-h incubation at ambient temperature, using the antibody covalently linked by the easily performed carbonyldiimidazole method, to microcrystalline cellulose. A detailed comparison of the accepted 0.125 mol/l citrate, pH 4.0, and an alternative 0.1 mol/l phosphate/8-anilinonaphthalene sulphonic acid, pH 7.4, diluent demonstrated similar precision (within-assay 10% to 5%; between-assay 11% to 6% over the range 100-1000 nmol/l cortisol) and recovery (median recoveries: phosphate 98.6%, citrate 99.5%). Linear regression analysis of human serum cortisol concentrations in comparison with the 'Amerlex' cortisol RIA kit gave the following results: citrate = 1.029. 'Amerlex' - 3.5 nmol/l (n = 80, r = 0.967); phosphate = 1.017, 'Amerlex' + 26.2 nmol/l (n = 80, r = 0.961). Phosphate, pH 7.4 diluent was adopted as the diluent of choice, since it was economical of antibody and maintained good precision over a wider working range of cortisol concentration.