What is the protocol for performing an outpatient dexamethasone (corticosteroid) suppression test?

Protocol for Outpatient Dexamethasone Suppression Test

The overnight 1-mg dexamethasone suppression test (DST) is the recommended protocol for outpatient screening of Cushing's syndrome, with dexamethasone administered at 11:00 PM and cortisol measured between 8:00-9:00 AM the following morning. 1

Standard Protocol for Overnight DST

1. Preparation:

   • Ensure patient is not taking medications that interfere with dexamethasone metabolism (particularly CYP3A4 inducers)
   • Women on estrogen-containing oral contraceptives may have false positive results 2

2. Test Administration:

   • Dose: 1 mg oral dexamethasone tablet 1, 3
   • Timing: Administer at 11:00 PM (23:00) 3
   • Alternative timing at 21:00 (9:00 PM) has been shown to be equally effective 4

3. Sample Collection:

   • Draw blood for serum cortisol measurement between 8:00-9:00 AM the following morning 3
   • Measure dexamethasone level along with cortisol to improve test interpretability 2

4. Result Interpretation:

   • Normal response: Cortisol suppression to <50 nmol/L (1.8 μg/dL) 5
   • Alternative cutoff: <80 nmol/L (3 μg/dL) can be used but may increase false positives 3
   • Failure to suppress suggests possible Cushing's syndrome

Special Considerations

For Obese Patients

• False positive rates of 2.3% have been reported with the 1-mg DST in obese patients 3
• Consider using a 2-mg overnight DST for obese patients, which has shown a lower false positive rate (2% vs 8% with 1-mg test) 5

Potential Pitfalls

• False positives may occur due to:

  • Rapid absorption/malabsorption of dexamethasone
  • Medications affecting dexamethasone metabolism
  • Increased corticosteroid binding globulin levels
  • Pseudo-Cushing's states (psychiatric disorders, alcohol use disorder, severe obesity) 1

• False negatives may occur due to:

  • Medications inhibiting dexamethasone metabolism
  • Cyclic Cushing's syndrome 1

Follow-up Testing

• If DST is positive (non-suppression of cortisol), confirm with additional tests:
  • 24-hour urinary free cortisol (UFC)
  • Late-night salivary cortisol (LNSC)
  • Low-dose dexamethasone suppression test (0.5 mg every 6 hours for 2 days) 3

Alternative Protocols

• For shift workers or patients with disrupted circadian rhythm, DST is preferred over other tests 1
• For suspected adrenal tumor, DST is recommended as the initial test 1
• For suspected cyclic Cushing's syndrome, LNSC may be more appropriate 1

The overnight 1-mg DST remains a reliable and convenient screening test for Cushing's syndrome in the outpatient setting, with good sensitivity and specificity when properly administered and interpreted.