Dexamethasone Dosing for Dexamethasone Suppression Test
For the overnight dexamethasone suppression test (DST), 1 mg of dexamethasone should be administered orally between 11:00 PM and midnight, with serum cortisol measurement at 8:00 AM the following morning. 1
Overnight 1-mg Dexamethasone Suppression Test Protocol
- 1 mg of dexamethasone is administered orally between 11:00 PM and midnight 1
- Serum cortisol is measured at 8:00 AM the following morning 1
- Normal response is defined as serum cortisol < 1.8 μg/dL (50 nmol/L) 1
- This test has high sensitivity (>90%) for detecting Cushing's syndrome 1
Interpretation of Results
- Serum cortisol < 1.8 μg/dL (50 nmol/L): Normal response, strongly predicts absence of Cushing's syndrome 1
- Serum cortisol 1.8-5.0 μg/dL (50-138 nmol/L): Possible autonomous cortisol secretion 1
- Serum cortisol > 5.0 μg/dL (138 nmol/L): Evidence of autonomous cortisol secretion, reduced test sensitivity 1
Alternative Timing Options
- Research shows that administering dexamethasone at 9:00 PM (21:00) instead of 11:00 PM (23:00) does not significantly affect the degree of cortisol suppression in healthy subjects 2
- This provides flexibility in test administration timing when needed 2
Potential Pitfalls and False Results
False Positive Results (Failure to suppress despite no Cushing's)
- Rapid absorption/malabsorption of dexamethasone due to increased gut transit time 1
- Chronic diarrhea or celiac disease 1
- Concomitant treatment with CYP3A4 inducers (e.g., phenobarbital, carbamazepine, St. John's wort) 1, 3
- Increased corticosteroid binding globulin levels from oral estrogens, pregnancy, or chronic active hepatitis 1
False Negative Results (Suppression despite Cushing's)
- Inhibition of dexamethasone metabolism by medications like fluoxetine, cimetidine, or diltiazem 1, 3
- Decreased corticosteroid binding globulin and albumin levels (e.g., nephrotic syndrome) 1
Special Considerations
- In obese patients, false positive rates may be higher with the 1 mg test 4
- Some studies suggest using a 2 mg overnight test in obese patients to reduce false positives from 8% to 2% 4
- Measuring dexamethasone levels concomitantly with cortisol can reduce the risk of false-positive results 1, 5
Low-Dose 2-Day Dexamethasone Suppression Test (LDDST)
For more definitive diagnosis when overnight test results are equivocal:
- Administer 0.5 mg dexamethasone orally every 6 hours (09:00,15:00,21:00,03:00) for 48 hours 3
- Measure cortisol at 0,24, and 48 hours 3
- Normal response is suppression to < 1.8 μg/dL (50 nmol/L) 3
The overnight 1-mg DST remains the standard screening test for Cushing's syndrome due to its high sensitivity, simplicity, and convenience compared to the more complex 2-day LDDST protocol.