Dexamethasone metabolism in dexamethasone suppression test suppressors andnonsuppressors

Background: Variable dexamethasone kinetics is a possible confound in the d examethasone suppression test, Modifications to include dexamethasone plasm a levels and specific dexamethasone "windows" have been proposed. Our study aims to validate our proposed dexamethasone windows in art independent sam ple of 121 subjects. Methods: We performed dexamethasone suppression tests in 162 subjects with mixed psychiatric diagnoses. Dexamethasone levels and beta-phase half-life of dexamethasone were computed for suppressors and nonsuppressors. Results: Dexamethasone levels were lower in nonsuppressors than in suppress ors. Dexamethasone levels correlated inversely with cortisol levels in the total sample, but were nonsignificant or weakly associated in those samples restricted to the windows. The beta-phase half-life of dexamethasone was s horter in nonsuppressors. The dexamethasone windows were validated at 3:00 PM and 10:00 PM. We propose 4.0 ng/mL as a revised upper limit of the 8:00 AM dexamethasone window. Conclusions: The plasma dexamethasone level is confirmed as a confound in t he dexamethasone suppression test through more rapid dexamethasone clearanc e in nonsuppressors, Application of dexamethasone windows will reduce this source of test variance. (C) 2000 Society of Biological Psychiatry.