Interpretation of 2-Day LDDST Results

Understanding LDDST Results

The Endocrine Society recommends that a cortisol decrease from 19 to 6 during a 2-day Low-Dose Dexamethasone Suppression Test (LDDST) indicates appropriate suppression of the hypothalamic-pituitary-adrenal axis, effectively ruling out Cushing's syndrome 1, 2
The LDDST is a key diagnostic test for confirming Cushing's syndrome, with cortisol suppression below 50 nmol/L (1.8 μg/dL) considered normal, as stated by the American Association of Clinical Endocrinologists 1, 3
In the 2-day LDDST protocol, dexamethasone is administered at 0.5 mg every 6 hours (09:00, 15:00, 21:00, 03:00) for 48 hours, with cortisol measurements at 0, 24, and 48 hours, according to the Endocrine Society 2
A post-LDDST cortisol level ≥50 nmol/L (≥1.8 μg/dL) is considered diagnostic for Cushing's syndrome with 95% sensitivity and 80% specificity, as reported by the American College of Endocrinology 4
A cortisol decrease from 19 to 6 represents significant suppression, indicating normal HPA axis function, as stated by the European Society of Endocrinology 3

The substantial decrease in cortisol levels (from 19 to 6) demonstrates appropriate negative feedback of the HPA axis in response to exogenous glucocorticoids, according to the American Association of Clinical Endocrinologists 3
This result is inconsistent with the pattern seen in Cushing's syndrome, where cortisol would remain elevated (≥50 nmol/L) despite dexamethasone administration, as reported by the Endocrine Society 2, 4

Initial screening tests, as recommended by the American College of Endocrinology, include 24-hour urinary free cortisol (UFC) measurements (3 collections) 1 and overnight 1-mg dexamethasone suppression test 3
The 2-day LDDST is also a recommended initial screening test, with suppression to <50 nmol/L (<1.8 μg/dL) considered a normal response, ruling out Cushing's syndrome, as stated by the Endocrine Society 1, 2, 4

False positive results (failure to suppress despite no Cushing's) can occur with medications affecting dexamethasone metabolism (CYP3A4 inducers like phenobarbital, carbamazepine), as reported by the European Society of Endocrinology 3
False negative results (suppression despite Cushing's) can occur with medications inhibiting dexamethasone metabolism (fluoxetine, cimetidine), according to the American Association of Clinical Endocrinologists 3