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

Clin Biochem. 2009 Jan;42(1-2):64-71. doi: 10.1016/j.clinbiochem.2008.09.115. Epub 2008 Oct 17.


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.

Design and methods: 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 (K(A)) 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 K(A) 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.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.
  • Validation Study

MeSH terms

  • Carrier Proteins / analysis*
  • Carrier Proteins / blood
  • Humans
  • Hydrocortisone / blood*
  • Mathematical Computing
  • Protein Binding
  • Sepsis / blood
  • Sepsis / metabolism
  • Serum Albumin / analysis*
  • Shock, Septic / blood
  • Shock, Septic / metabolism
  • Ultrafiltration


  • Carrier Proteins
  • Serum Albumin
  • cortisol binding globulin
  • Hydrocortisone