The lower limits of dexamethasone window in Chinese depressives

Background: Previous studies have shown wide variation in plasma dexamethasone (DEX) concentrations following a standard 1-mg dexamethasone suppression test (DST), and significantly lower DEX concentrations in DST nonsuppressors compared with suppressors, suggesting that DEX pharmacokinetics/bioavailability is an important variable associated with DST nonsuppression. Methods: To determine the effect of plasma DEX levels on the DST in Chinese depressives, we measured plasma DEX and post-DEX cortisol levels at 4:00 in a group of 50 depressed outpatients, 28 anxiety outpatients, and 33 normal subjects during the course of 1-mg oral overnight DST. Results: We found a significant difference in the plasma DEX levels between DST nonsuppressors and suppressors in the depression group and overall subject population, and a significant negative correlation between the plasma DEX and cortisol levels in the depression, anxiety, and total groups. Within a DEX “window”, the DST performance was enhanced, whereas the relationships between plasma DEX and post-DEX cortisol levels remained equally strong. Conclusions: Our findings support a relationship between plasma DEX and post-DEX cortisol levels, a relationship that might be superimposed on the hypothalamic–pituitary–adrenal axis. Comparing our “window” range with those of previous studies, we suggest that Chinese depressives may have lower limits of plasma DEX window, and that ethnicity may be an intervening variable in both DST response and pharmacokinetics of DEX.