Dexamethasone metabolism in dexamethasone suppression test suppressors and nonsuppressors.
BACKGROUND
Variable dexamethasone kinetics is a possible confound in the dexamethasone suppression test. Modifications to include dexamethasone plasma levels and specific dexamethasone "windows" have been proposed. Our study aims to validate our proposed dexamethasone windows in an independent sample 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 suppressors. 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 shorter 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 the dexamethasone suppression test through more rapid dexamethasone clearance in nonsuppressors. Application of dexamethasone windows will reduce this source of test variance.
Cassidy
F
F
Department of Psychiatry and Behavioral Sciences, Duke University Medical Center, Durham, North Carolina 27710, USA.
Ritchie
J C
JC
Verghese
K
K
Carroll
B J
BJ
D000304:Adrenal Cortex Function Tests; D000328:Adult; D003907:Dexamethasone / Q000097:blood / Q000493:pharmacokinetics*; D005260:Female; D005938:Glucocorticoids / Q000097:blood / Q000493:pharmacokinetics*; D006207:Half-Life; D006801:Humans; D006854:Hydrocortisone / Q000097:blood; D008297:Male; D001523:Mental Disorders / Q000097:blood / Q000175:diagnosis*; D008875:Middle Aged; D011237:Predictive Value of Tests; D015203:Reproducibility of Results
D005938:Glucocorticoids; D003907:Dexamethasone; D006854:Hydrocortisone