Will Exercise or Coffee Interfere with the 1 mg Dexamethasone Suppression Test?
No, neither exercise nor coffee consumption will directly interfere with the 1 mg dexamethasone suppression test (DST) itself, but caffeine should be avoided for 24 hours before testing as a standard precaution, and vigorous exercise should be avoided for 48 hours prior to ensure accurate baseline cortisol assessment. 1
Caffeine Considerations
Caffeine avoidance for 24 hours before the DST is recommended as a standardized pre-test condition, though the evidence specifically linking caffeine to false-positive DST results is limited 1
The rationale for caffeine restriction relates to its potential effects on the hypothalamic-pituitary-adrenal (HPA) axis activation rather than direct interference with dexamethasone metabolism 1
Caffeine does not affect dexamethasone absorption or metabolism through CYP3A4 pathways, which is the primary concern for DST accuracy 1, 2
Exercise Considerations
Vigorous physical activity should be avoided for 48 hours before the DST to prevent transient HPA axis activation that could elevate baseline cortisol levels 1
Exercise itself does not interfere with dexamethasone's pharmacologic action or its metabolism, but intense activity can temporarily increase cortisol secretion 1
The concern is ensuring stable baseline cortisol status rather than direct test interference—patients should be well-rested when taking dexamethasone at 11 PM and when having morning cortisol measured 1
What Actually Interferes with the DST
The key factors that genuinely compromise DST accuracy are:
CYP3A4 inducers (phenobarbital, carbamazepine, St. John's wort) that accelerate dexamethasone metabolism, leading to false-positive results 1
CYP3A4 inhibitors (fluoxetine, cimetidine, diltiazem) that slow dexamethasone metabolism, potentially causing false-negative results 1
Oral estrogens that increase corticosteroid-binding globulin (CBG), elevating total cortisol measurements 1
Malabsorption conditions (celiac disease, chronic diarrhea) affecting dexamethasone absorption 1
Practical Testing Protocol
Administer 1 mg dexamethasone orally between 11 PM and midnight 1, 3
Normal response is cortisol <50 nmol/L (1.8 μg/dL), which excludes Cushing's syndrome with high sensitivity (>90%) 1, 3
Consider measuring dexamethasone levels concomitantly with cortisol to identify inadequate drug absorption (threshold ≥4.5 nmol/L indicates adequate levels) 1, 4, 5
Common Pitfall to Avoid
The most important pitfall is not recognizing medications that affect CYP3A4 metabolism—these have far greater impact on DST accuracy than lifestyle factors like caffeine or exercise 1, 2. Always obtain a complete medication history, including over-the-counter supplements and herbal products, before interpreting DST results 1, 2.