Radioimmunoassay for serum cortisol with 125I-labeled ligand: comparison of three methods.

We evaluated a previously modified double-antibody radioimmunoassay for serum cortisol. It was compared with the conventional double-antibody method that includes the usual extraction step, and also with an antibody-coated tube method. In this modified method, cortisol was released from its binding globulin by enzymatic degradation rather than by extraction with ether, and a preincubated mixture of first and second antibody was used to separate antibody-bound cortisol from free. These two steps shortened total asssay time significantly. Results still correlated well (r = 0.87) with results by the conventional melthod, but the antibody-coated tube method gave lower results (r = 0.61). Because of its good correlation with the conventional method, this method was thought to be more accurate. In 52 normal subjects, mean cortisol concentrations at 0800 and 1700 hours were 161 +/- 52 (SD) mug/liter and 91 +/- 27 mug/liter, respectively. In 16 normal subjects, cortisol values before and after dexamethasone treatment (1 mg at midnight) were 134 +/- 53 mug/liter and less than 20 mug/liter. In the same subjects, cortisol concentrations before and 30 min and 60 min after Cortrosyn (synthetic corticotropin1-24, 0.25 mg) administration were 103 +/- 25, 205 +/- 45 and 223 +/- 51 mug/liter, respectively.