An Albumin-Derived Free Cortisol Index May Be a Practical Surrogate for Free Plasma Cortisol Measurements

Abstract

Eighty percent to 90% of circulating cortisol is bound to cortisol-binding globulin (CBG) and albumin; only a small fraction is in the free, biologically active form. Traditionally, total serum cortisol measurements have been used to evaluate adrenal gland function. Accurate assessment, however, is difficult in severely ill patients because CBG falls rapidly and serum albumin levels are frequently low. Recently, direct measurements of free cortisol and a free cortisol index (FCI), derived by dividing the total cortisol by the CBG, have been shown to more accurately reflect adrenal gland function than the level of total cortisol in severely ill patients. Free cortisol levels are usually measured in reference laboratories and results arrive too late to be of practical value and CBG measurements are not generally available. In this study, we compared an FCI index, derived by dividing the total cortisol by the serum albumin, with free cortisol measured by equilibrium dialysis in 11 women and 7 men before and 24 hours after surgery. Since the correlation of FCI (Alb) to free cortisol was good, our pilot study suggests that the total cholesterol/albumin ratio may provide a surrogate measure for free cortisol that is readily available in most clinical laboratories. Further studies in a larger number of patients, especially those with hypoalbuminemia, however, are needed before the FCI (Alb) can be used confidently in critically ill patients.