Dexamethasone Suppression Test Protocol
For the overnight dexamethasone suppression test, administer 1 mg of dexamethasone orally between 11 PM and midnight (2300-2400 hours), then draw blood for cortisol measurement at 8-9 AM the following morning. 1, 2
Test Administration Details
The overnight 1-mg dexamethasone suppression test (DST) is one of the recommended first-line screening tests for Cushing's syndrome, with sensitivity above 90% 1. The protocol is straightforward:
- Timing of dexamethasone: Give 1 mg orally between 2300-2400 hours (11 PM-midnight)
- Timing of blood draw: Collect serum cortisol at 0800-0900 hours (8-9 AM) the next morning
- Normal response: Serum cortisol <1.8 μg/dL (50 nmol/L) 1, 2
Interpretation Thresholds
The most recent Lancet Diabetes & Endocrinology guideline (2021) recommends using <1.8 μg/dL (50 nmol/L) as the cutoff for normal suppression 1. This provides optimal sensitivity for detecting Cushing's syndrome. Higher cutoff points like 5 μg/dL (138 nmol/L) reduce sensitivity and should be reserved for specific contexts like evaluating adrenal incidentalomas 1.
Critical Considerations to Avoid False Results
Factors Causing False Positives:
- CYP3A4 inducers (phenobarbital, carbamazepine, St. John's wort) accelerate dexamethasone metabolism 1
- Increased CBG levels from oral estrogens, pregnancy, or chronic active hepatitis elevate total cortisol 1
- Malabsorption conditions (celiac disease, chronic diarrhea) reduce dexamethasone absorption 1
Factors Causing False Negatives:
- CYP3A4 inhibitors (fluoxetine, cimetidine, diltiazem) increase dexamethasone bioavailability 1
- Decreased CBG/albumin (nephrotic syndrome) lowers total cortisol 1
Enhanced Accuracy Strategy
Measuring dexamethasone levels simultaneously with cortisol significantly improves diagnostic accuracy by identifying patients with inadequate drug exposure versus true cortisol excess 1, 3, 4. The recommended dexamethasone cutoff is ≥3.48 nmol/L (1.3 ng/mL) to confirm adequate drug exposure 3, 4.
Important Caveats
When NOT to Use This Test:
- Pregnancy: Use 24-hour urinary free cortisol instead; avoid dexamethasone testing 2
- Patients on antiepileptic drugs: These enhance dexamethasone clearance; use non-suppressed cortisol measurements instead 2
- Cyclic Cushing's syndrome: Prefer urinary free cortisol or late-night salivary cortisol over DST 2
- Night-shift workers: Disrupted circadian rhythm invalidates the test 1
Limitations in Mild/Episodic Disease:
Recent research demonstrates that patients with mild or periodic Cushing's syndrome frequently suppress normally to 1 mg dexamethasone (sensitivity as low as 18-41% in some studies) 5, 6. If clinical suspicion remains high despite normal suppression, proceed with alternative testing such as late-night salivary cortisol or 24-hour urinary free cortisol 1, 2.
Alternative: 48-Hour Low-Dose Test
For improved specificity, particularly in patients with psychiatric conditions, obesity, or alcoholism that can activate the HPA axis, consider the 48-hour low-dose test: 0.5 mg dexamethasone every 6 hours for 48 hours (at 0900,1500,2100,0300 hours), with cortisol measured 6 hours after the last dose 2. This has better specificity than the overnight test but requires more patient compliance 2.