Dexamethasone Suppression Test Dosing
For the standard overnight dexamethasone suppression test, administer 1 mg of dexamethasone orally at 11:00 PM with cortisol measurement the following morning between 8:00-9:00 AM. 1, 2, 3
Standard Overnight Dexamethasone Suppression Test Protocol
Dosing:
- Standard dose: 1 mg dexamethasone orally
- Timing: Administered at 11:00 PM (23:00)
- Cortisol measurement: 8:00-9:00 AM the following morning
Interpretation:
- Normal response: Cortisol suppression to <80 nmol/L (3 μg/dL)
- Abnormal response (suggesting Cushing's syndrome): Cortisol ≥80 nmol/L
Alternative Timing and Dosing Options
Administration at 21:00 (9:00 PM) produces similar suppression results as the standard 23:00 timing, which may be more convenient for outpatient testing 4
Different dexamethasone doses have been studied:
- 0.25 mg: Less effective suppression
- 0.5 mg: Similar suppression to 1 mg
- 1.0 mg: Standard dose with established cutoffs 4
Special Considerations
Factors Affecting Test Accuracy:
- Dexamethasone bioavailability can affect results; approximately 6% of patients may not achieve adequate dexamethasone levels despite proper administration 5
- Women taking estrogen-containing oral contraceptives may have false positive results due to altered dexamethasone metabolism 1
- False positives can occur in patients with:
- Severe obesity
- Uncontrolled diabetes
- Pregnancy
- Polycystic ovary syndrome
- Psychiatric disorders
- Alcohol use disorder 1
Test Limitations:
- Sensitivity is significantly reduced in patients with mild or episodic hypercortisolism, with sensitivity as low as 18-41% depending on the cutoff used 6
- For patients with suspected mild or periodic Cushing's syndrome, additional testing methods should be considered
Pre-Test Preparation
- Patients should avoid:
- Eating, drinking, smoking, or brushing teeth 1-2 hours before collection
- Strenuous physical activity for 24-48 hours
- Smoking/vaping/nicotine products for 4 hours
- Alcohol consumption for 12 hours before sampling 1
Alternative Testing Methods
- Late-night salivary cortisol (LNSC): >90% sensitivity, 100% specificity
- 24-hour urinary free cortisol (UFC): Reflects overall cortisol production
- Plasma ACTH: Helps determine ACTH-dependent vs. independent cortisol excess 1
The overnight dexamethasone suppression test remains a valuable screening tool for Cushing's syndrome with high sensitivity in most cases, though clinicians should be aware of its limitations in mild or episodic disease.