Validation of a simple method of estimating plasma free cortisol: Role of cortisol binding to albumin

Objectives:To develop, optimize, and validate a generalized mass action, equilibrium solution that incorporates measured concentrations of albumin as well as cortisol binding globulin (CBG) to estimate free cortisol.Designandmethods:Free cortisol was estimated by Coolens method or by cubic equilibrium equation and compared to measured free cortisol, determined by ultrafiltration method, in subjects with septic shock (n = 45), sepsis (n = 19), and healthy controls (n = 10) at 0, 30, and 60 min following administration of cosyntropin (250 mcg). The data set also included repeat testing in 30 subjects following recovery from sepsis/septic shock. The equilibrium dissociation constant for cortisol binding to albumin (KA) was optimized by non-linear regression. The cubic equilibrium solution was also used to model the influence of cortisol, CBG, and albumin concentration on free cortisol.Results:Compared to measured free cortisol, the cubic solution, using an optimized KA of 137,800 nM, was less biased than Coolens solution, with mean percent error of − 23.0% vs. − 41.1% (paired t test, P < 0.001). Standard deviation values were also significantly lower (Wilks' test, P < 0.001) for the cubic solution (SD 35.8% vs. 40.8% for cubic vs. Coolens, respectively). Modeling studies using the cubic solution suggest an interaction effect by which low concentrations of CBG and albumin contribute to a greater increase in free cortisol than the sum of their independent effects.Conclusions:Mass action solutions that incorporate the measured concentration of albumin as well as CBG provide a reasonably accurate estimate of free cortisol that generalizes to conditions of health as well as a setting of hypercortisolism and low CBG and albumin concentrations associated with septic shock. Modeling studies emphasize the significant contribution of albumin deficiency and albumin-bound cortisol under conditions of CBG-deficiency, and identify a synergistic effect by which combined CBG and albumin deficiency contribute to elevation of free cortisol in septic shock.