Use of saliva for monitoring unbound free cortisol levels in serum.

In order to verify the clinical usefulness of saliva in assessing the adrenocortical function, we measured saliva cortisol levels by a radioimmunoassay after extraction with dichloromethane, and compared the results with the levels of serum unbound cortisol determined by the method of equilibrium dialysis. Paired samples of saliva and serum were obtained from 10 healthy male volunteers. Morning levels of saliva cortisol and serum unbound cortisol were 0.99 +/- 0.42 and 1.56 +/- 0.54 microgram/100 ml, respectively, where serum total cortisol was 19.28 +/- 3.56 microgram/100 ml. A well-defined diurnal rhythm and a greater response to ACTH stimulation were observed in saliva cortisol than in serum total cortisol. Kinetic study of injected cortisol revealed almost identical values for the compartments of saliva cortisol and serum unbound cortisol. The correlation coefficient (r) between the levels of saliva cortisol and serum unbound cortisol was 0.893 (p less than 0.001, n = 150). From these results we concluded that the measurement of saliva cortisol can be used sufficiently to monitor unbound free concentrations in serum.