Interpretation of dexamethasone suppression test (DST) may be influenced by dexamethasone absorption and metabolism and by the altered cortisol binding.

We aimed to determine the normal ranges of free cortisol during DST in participants without adrenal disorders and to identify the population of patients where post-DST free cortisol measurements add value to the diagnostic workup.

Cross-sectional study conducted in a tertiary medical center.

Adult volunteers without adrenal disorders (n = 168; 47 women on oral contraceptive therapy [OCP], 66 women not on OCP, 55 men) and patients undergoing evaluation for hypercortisolism (n = 196; 16 women on OCP).

Post-DST dexamethasone and free cortisol (mass spectrometry) and total cortisol (immunoassay).

Reference range for post-DST free cortisol, diagnostic accuracy of post-DST total cortisol.

Adequate dexamethasone concentrations (≥0.1 mcg/dL) were seen in 97.6% volunteers and 96.3% patients. Only 25.5% of women volunteers on OCP had abnormal post-DST total cortisol (>1.8 mcg/dL). In volunteers, the upper post-DST free cortisol range was 48 ng/dL in men and women not on OCP, and 79 ng/dL in women on OCP. When compared with post-DST free cortisol, diagnostic accuracy of post-DST total cortisol was 87.3% (95% CI, 81.7-91.7); all false-positive results occurred in patients with post-DST cortisol between 1.8 and 5 mcg/dL. OCP use was the only factor associated with false-positive results (21.1% vs 4.9%, P = 0.02).

Post-DST free cortisol measurements are valuable in patients with optimal dexamethasone concentrations and post-DST total cortisol between 1.8 and 5 mcg/dL.