The dexamethasone suppression index: enhancement of DST diagnostic utility for depression by expressing serum cortisol as a function of serum dexamethasone.

The authors sought to determine whether the performance of the dexamethasone suppression test (DST) could be enhanced by expressing cortisol as a function of dexamethasone. Because cortisol concentration is a function of the reciprocal of dexamethasone concentration, this relationship was approximated by calculating the product of cortisol and dexamethasone as a dexamethasone suppression index. Preliminary assessment of test performance measures (sensitivity, specificity, and predictive power) showed that use of the dexamethasone suppression index was an improvement over the use of cortisol levels alone. Factoring dexamethasone levels into post-dexamethasone cortisol level measures may enhance the utility of neuroendocrine assessment in psychiatry.