MON-169 Diagnostic Utility of Free Cortisol in Dexamethasone Suppression Test. A Prospective Study in Healthy Subjects

OBJECTIVE: Low dose dexamethasone suppression testing (DST) is standard of care in patients with adrenal incidentalomas or suspected endogenous hypercortisolism. False positive total serum cortisol (TC) results occur due to poor absorption, rapid metabolism or estrogen use. Free serum cortisol (FC) measurement is an alternative, but optimal cut-offs are unknown. We aimed to establish the optimal serum dexamethasone concentrations (DEXA) to interpret TC and FC results and identify reasons for discrepancies between TC and FC values in healthy male and female (with and without oral contraceptive therapy, OCT) subjects. METHODS: Single center prospective study of healthy subjects >17 years old undergoing assessment with DST between 2016 and 2019. Measurement of FC and DEXA was performed by tandem mass spectrometry and TC by immune-enzymatic assay at 8 AM following overnight administration of 1 mg dexamethasone at 11pm. Subjects were excluded if they had a known adrenal adenoma, any form of endogenous cortisol excess, or exogenous steroid use. RESULTS: DST was performed in 165 healthy volunteers, median age of 29.5 (18-74) years; 53 men (32%) and 112 (67%) women, median BMI 25 (18-42) kg/m2, 47 (42%) of which were taking OCT (median daily ethinyl-estradiol dose of 30 (20-35) mcg). The median DEXA was 0.34 (0.09-1.12) mcg/dl, median TC was 0.8 (0.25-15.7) mcg/dl and median FC was 24 (4-714) ng/ml. TC and FC measurements were highly correlated (r2=0.89, p1.8 was higher when DEXA was 0.2mcg/dl (2/101, 2%),(p