Interpretation of Abnormal Dexamethasone Suppression Test is Enhanced With Use of Synchronous Free Cortisol Assessment

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The Journal of clinical endocrinology and metabolism


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

OBJECTIVE: 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 work up.

DESIGN AND SETTING: Cross-sectional study conducted in a tertiary medical center.

PARTICIPANTS: 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).

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

MAIN OUTCOME MEASURES: Reference range for post-DST free cortisol, diagnostic accuracy of post-DST total cortisol.

RESULTS: 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 to 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).

CONCLUSIONS: 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.

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ePub ahead of print