Free cortisol index: a rapid and simple estimation of free cortisol in human plasma.

A rapid and simple method for the estimation of free (nonprotein-bound) cortisol in human plasma is described. Total plasma cortisol (F) was determined by a competitive protein binding assay. An estimate of the saturation of Corticosteroid Binding Globulin was obtained by the use of "charcoal cortisol uptake" (CFU). "Charcoal cortisol uptake ratio" (CFUR) was defined as the ratio of the CFU of the unknown plasma to that of a reference serum pool. Free cortisol index (FFI) was calculated as (total plasma F) times (CFUR). Percent free cortisol and free cortisol concentration (FF) was determined by equilibrium dialysis at 37 C. 169 plasma samples with an F range of 0.8-61.2 mug/100 ml from 18 normal, 10 ACTH-stimulated, 17 estrogen-treated and 12 pregnant subjects were analyzed. A highly significant correlation between FFI and FF was found (r = 0.952). Inter-assay coefficients of variation were 7% for total plasma F and 3.5% for CFU. The method allows the rapid processing of large numbers of samples and can be performed on 0.2 ml of plasma.