Dexamethasone Suppression Test Guidelines

Test Protocol

The Endocrine Society recommends administering 1 mg of dexamethasone orally between 11:00 PM and midnight, with serum cortisol measurement at 8:00 AM the following morning 1, 2
Serum cortisol is measured at 8:00 AM the following morning, with a normal response defined as serum cortisol < 1.8 μg/dL (50 nmol/L) 1, 2

Serum cortisol < 1.8 μg/dL (50 nmol/L) is considered a normal response, strongly predicting the absence of Cushing's syndrome, with high sensitivity (>90%) for detecting Cushing's syndrome 1, 2
Serum cortisol > 5.0 μg/dL (138 nmol/L) is evidence of autonomous cortisol secretion, with reduced test sensitivity 1, 3

Rapid absorption or malabsorption of dexamethasone due to increased gut transit time can cause false positive results 1
Concomitant treatment with CYP3A4 inducers (e.g., phenobarbital, carbamazepine, St. John's wort) can cause false positive results 1, 4
Inhibition of dexamethasone metabolism by medications like fluoxetine, cimetidine, or diltiazem can cause false negative results 1, 4

Measuring dexamethasone levels concomitantly with cortisol can reduce the risk of false-positive results 1

The 2-day LDDST protocol involves administering 0.5 mg dexamethasone orally every 6 hours for 48 hours, with cortisol measurement at 0, 24, and 48 hours 4
Normal response is suppression to < 1.8 μg/dL (50 nmol/L) 4

Pseudo-Cushing's states, such as depression, alcoholism, and obesity, can lead to false positive results, and the combined low-dose dexamethasone-CRH test (Dex-CRH) may help distinguish between ACTH-dependent Cushing's syndrome and pseudo-Cushing's states, as recommended by The Endocrine Society 5
Initial screening for Cushing's syndrome should be based on clinical suspicion, with low suspicion cases starting with late-night salivary cortisol (LNSC) and intermediate/high suspicion cases performing 2-3 screening tests (LNSC, 24h UFC, overnight DST), according to guidelines from The Endocrine Society 5, 6

Interpretation of screening results should follow a step-wise approach, with normal results indicating Cushing's syndrome is unlikely, abnormal results requiring repeat screening tests, and persistently abnormal results proceeding to ACTH measurement, as outlined by The Endocrine Society 6
For ACTH-dependent cases, pituitary MRI is recommended, and for cases with pituitary adenoma ≥10 mm, presumed Cushing's disease, with inferior petrosal sinus sampling (IPSS) considered for adenomas <6 mm or no adenoma, as suggested by The Endocrine Society 5, 6

The Endocrine Society recommends that when post-dexamethasone cortisol is borderline (1.8-5.0 μg/dL), additional evaluation rather than escalation to high-dose testing should be performed, as this range represents a diagnostic gray zone where the test is neither clearly normal nor definitively diagnostic of overt Cushing's syndrome 7

The American College of Endocrinology suggests measuring the post-test dexamethasone level concomitantly with cortisol to identify false-positive results due to rapid dexamethasone metabolism or malabsorption, with a dexamethasone level <1.8 ng/mL (4.6 nmol/L) suggesting inadequate drug exposure, invalidating the test result 7
The Endocrine Society recommends obtaining 2-3 additional screening tests, including 24-hour urinary free cortisol (UFC), late-night salivary cortisol (LNSC), and repeat overnight 1 mg DST, to account for intra-patient variability and cyclic Cushing's syndrome 7

The American Association of Clinical Endocrinologists advises against using the 8 mg DST for diagnosis, as this test differentiates pituitary from ectopic ACTH sources only after Cushing's syndrome is confirmed, with a strength of evidence level of high 8

The Endocrine Society suggests that in patients with mild hypercortisolism and low clinical suspicion, monitoring for 3-6 months to see if symptoms resolve is recommended, as treatment of underlying conditions can restore normal HPA axis function and cortisol levels, with a moderate level of evidence 7

The American Urological Association recommends that all patients with adrenal incidentalomas require hormone screening, including overnight 1-mg DST, to detect subclinical Cushing's syndrome 9, 10
The Endocrine Society suggests that management of subclinical Cushing's syndrome in patients with adrenal incidentalomas requires shared decision-making, weighing patient preference, comorbidity severity, and surgical risk, as progression to overt Cushing's is rare 9, 10
The American College of Surgeons recommends optimizing comorbid conditions preoperatively in patients with confirmed Cushing's syndrome scheduled for adenoma resection, including controlling diabetes and hypertension, and providing prophylactic antibiotics 9