Radioimmunoassayfor Serum Cortisol with 1251-Labeled Ligand:Comparison of Three Methods

We evaluated a previously modified double-antibody ra-dioimmunoassay 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 anti-body-bound cortisol from free. These two steps shortened total assay time significantly. Results still 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 hours were 161 ± 52 (SD) tg/liter and 91 ± 27 sg/liter, respectively. In 16 normal subjects, cortisol values before and after dexamethasone treatment (1 mg at mid-night) were 134 ± 53 tg/liter and <20 ig/Iiter. In the same subjects, cortisol concentrations before and 30 mm and 60 mm after Cortrosyn (synthetic corticotropin1.24, 0.25 mg) administration were 103 ± 25, 205 ± 45 and 223 ± 51 pg/liter, respectively. AddItional Keyphrases: modified double-antibody method #{149}coated-tube method #{149}cross reaction of antiserum #{149}midnight dexamethasone suppression test #{149}Cor-trosyn stimulation test and interassay variations #{149}inter-method comparison #{149}cortisol binding globulin #{149}normal values Plasma cortisol (1 1$, 17a,2 1-trihydroxy-4-preg-nene-3,20-dione) has been commonly measured by fluorometric, double isotope derivative, and competitive protein-binding techniques. Although these methods are still used in some laboratories, they lack specificit