Radioimmunoassay of urinary free cortisol

The radioimmunoassay for urinary free cortisol described in this paper is simple, rapid, and reproducible. The method uses a commercially available antibody preparation and is performed in two steps. The first step includes an extraction and a column purification to remove materials antigenically similar to cortisol from the urine. The second step is the radioimmunoassay using dextran-coated charcoal to separate bound and unbound cortisol. 3H-cortisol is added prior to any mechanical manipulation to allow calculation of analytical recovery for the purification procedure. The coefficient of variation for interassay determinations was a maximum of 10.3% and for intraassay determinations a maximum of 5.7%. Analytical recovery averaged 97.7%. One technician can analyze 100 samples per week.