RADIOIMMUNOASSAY FOR SERUM CORTISOL WITH I-125-LABELED LIGAND - COMPARISON OF 3 METHODS

A previously modified double-antibody radioimmunoassay for serum cortisol was evaluated. It was compared with the conventional double-antibody method that includes the usual extraction step, and 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 1st and 2nd antibody was used to separate antibody-bound cortisol from free. These 2 steps shortened total assay time significantly. Results correlated well (r = 0.87) with results by the conventional method, 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 h were 161 .+-. 52 (SD) .mu.g/l and 91 .+-. 27 .mu.g/l, respectively. In 16 normal [human] subjects, cortisol values before and after dexamethasone treatment (1 mg at midnight) were 134 .+-. 53 .mu.g/l and < 20 .mu.g/l. 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 .mu.g/l, respectively. [Rabbit antibodies were used].