Measurement of salivary cortisol concentration in the assessment of adrenal function in critically ill subjects: A surrogate marker of the circulating free cortisol

Methods: Baseline and cosyntropin- stimulated serum (total and free) and salivary cortisol concentrations were measured, in the early afternoon, in 51 critically ill patients and healthy subjects. Patients were stratified according to their serum albumin at the time of testing: those whose serum albumin levels were 2.5 gm/dl or less vs. others whose levels were greater than 2.5 gm/dl. Results: Baseline and cosyntropin- stimulated serum free cortisol levels were similar in the two groups of critically ill patients and were severalfold higher (P < 0.001) than those of healthy subjects. Similarly, baseline and cosyntropin-stimulated salivary cortisol concentrations were equally elevated in the two critically ill patient groups and were severalfold higher (P < 0.001) than those of healthy subjects. Salivary cortisol concentrations correlated well with the measured serum free cortisol levels. Conclusions: Salivary cortisol measurements are simple to obtain, easy to measure in most laboratories, and provide an indirect yet reliable and practical assessment of the serum free cortisol concentrations during critical illnesses. The concentrations of the two measures of unbound cortisol determined in two different body fluids correlated very well, regardless of the serum protein concentrations. Measurements of salivary cortisol can serve as a surrogate marker for the free cortisol in the circulation.