Does a dexamethasone suppression test require fasting?

Dexamethasone Suppression Test and Fasting Requirements

The overnight 1-mg dexamethasone suppression test (DST) does not require fasting, but patients should avoid eating a heavy meal within 2-3 hours of the test.

Test Protocol and Administration

The overnight 1-mg DST is one of the primary screening tests for Cushing's syndrome, with high sensitivity when properly administered. According to the 2021 consensus guidelines from The Lancet Diabetes and Endocrinology 1:

• Dexamethasone (1 mg) is administered between 2300h and midnight
• Serum cortisol is measured at 0800h the following morning
• Normal response: cortisol < 1.8 μg/dL (50 nmol/L)
• A negative result strongly predicts absence of Cushing's syndrome

Fasting Status and Test Preparation

While strict fasting is not required for the DST, certain dietary considerations are important:

• No specific fasting requirement is mentioned in the 2021 Cushing's disease guideline 1
• For exercise stress testing (as a comparison for medical testing protocols), patients are advised "not to eat a heavy meal within 2-3 hours of the test" 1
• For other endocrine tests like PET scans that involve glucose metabolism, fasting is explicitly required 1

Factors That May Affect Test Results

Several factors can influence DST results, leading to false positives or negatives 1, 2:

False Positives (indicating Cushing's syndrome when not present):

• Rapid absorption/malabsorption of dexamethasone
• Chronic diarrhea or celiac disease
• Medications that induce CYP3A4 (phenobarbital, carbamazepine, St. John's wort)
• Increased corticosteroid binding globulin levels from:
  • Oral estrogens
  • Pregnancy
  • Chronic active hepatitis

False Negatives (missing Cushing's syndrome):

• Medications that inhibit dexamethasone metabolism (fluoxetine, cimetidine, diltiazem)
• Decreased corticosteroid binding globulin and albumin levels (e.g., nephrotic syndrome)

Best Practices for Test Accuracy

To improve test accuracy:

• Consider measuring dexamethasone levels concomitantly with cortisol 1
• Use laboratory-specific ranges of expected values 1
• For obese patients, a 2-mg overnight DST may be more accurate than the standard 1-mg test (2% vs. 8% false-positive rate) 3
• In patients with mild or episodic hypercortisolism, be aware that the sensitivity of the 1-mg DST may be lower than traditionally reported 4

Special Considerations

• For suspected cardiac inflammation or when evaluating structures near the heart, specific myocardial suppression protocols requiring prolonged fasting (at least 12 hours) and dietary modifications are recommended 1
• In diabetic patients, scheduling the test for late morning (approximately 4 hours after breakfast) may help achieve lower blood glucose levels 1

Summary

While the overnight DST does not specifically require fasting, avoiding heavy meals before the test is advisable. The most important factors for accurate results are proper timing of dexamethasone administration, awareness of medications that might interfere with the test, and correct interpretation of results based on established cutoff values.