Overnight Dexamethasone Suppression Test: A Reliable Screen for Cushing's Syndrome in the Obese

Objective: Reevaluation of the validity of the 1‐mg overnight dexamethasone suppression test (ODST) as a screening test for Cushing's syndrome in obese patients. Research Methods and Procedures: Eighty‐six obese patients (body mass index, 30 to 53 kg/m 2 ) that were referred to a general endocrine outpatient clinic for evaluation of simple obesity, diabetes mellitus, hypertension, polycystic ovary disease, or pituitary tumor. One milligram dexamethasone was administered orally at 11:00 pm , and serum cortisol levels were measured the following morning between 8:00 am and 9:00 am . Suppression of serum cortisol to <80 nM (3 μg/dL) was chosen as the cut‐off point for normal suppression. Patients with serum cortisol levels ≥80 nM were evaluated for Cushing's syndrome. Results: Suppression of morning cortisol levels to <80 nM occurred in 79 of the 86 obese patients. Seven patients had serum cortisol levels higher than 80 nM; five were eventually diagnosed with Cushing's syndrome and two were considered false positive results in view of normal 24‐hour free urinary cortisol and normal suppression on a low dose dexamethasone suppression test (0.5 mg of dexamethasone every 6 hours for 2 days). We found a false positive rate of 2.3% for the ODST using a cut‐off serum cortisol of 80 nM. Discussion: The ODST is a valid screening test for Cushing's syndrome in the obese population. The false positive rate was 2.3%, even when using a strict cut‐off serum cortisol of 80 nM. Abnormal cortisol suppression in obese patients should be investigated and not be considered false positive results.