Should You Repeat an Equivocal Dexamethasone Suppression Test?
Yes, repeat the dexamethasone suppression test when results are equivocal, and perform at least 2-3 additional screening tests (late-night salivary cortisol and 24-hour urinary free cortisol) to establish or exclude Cushing's syndrome. 1
Algorithmic Approach to Equivocal DST Results
Step 1: Verify Technical Factors First
Before repeating the DST, rule out causes of false results:
Check for False Positives:
- Measure dexamethasone levels simultaneously with cortisol using laboratory-specific ranges 1
- Review medications: CYP3A4 inducers (phenobarbital, carbamazepine, St. John's wort) accelerate dexamethasone metabolism 1, 2
- Assess for increased corticosteroid binding globulin: oral estrogens, pregnancy, chronic active hepatitis 1
- Consider malabsorption issues: chronic diarrhea, celiac disease, rapid gut transit 1
Check for False Negatives:
- Review for CYP3A4 inhibitors: fluoxetine, cimetidine, diltiazem (these increase dexamethasone bioavailability) 1
- Assess for decreased binding proteins: nephrotic syndrome, low albumin 1
- Indomethacin can cause false negative results 2
Step 2: Interpret Your Cortisol Cutoffs
The guideline provides clear thresholds 1:
- <1.8 μg/dL (50 nmol/L): Normal response, excludes Cushing's syndrome
- 1.8-5 μg/dL: Equivocal zone requiring further evaluation
- >5 μg/dL (138 nmol/L): Suggests dysregulated cortisol secretion
Step 3: Perform Complementary Testing (Don't Just Repeat DST Alone)
The guideline explicitly recommends at least 2-3 tests of each type 1:
- Late-night salivary cortisol (LNSC): Highest specificity among screening tests; perform 2-3 collections at usual bedtime
- 24-hour urinary free cortisol (UFC): Perform 2-3 collections to account for 50% intra-patient variability
- Consider combined LDDST-CRH test: May serve as a rule-out procedure in indeterminate cases, particularly when repeat testing or follow-up is unavailable 3
Step 4: Serial Testing is Safe and Informative
Repeated DST does not produce false positives from serial testing itself 4. Weekly repetitions in healthy volunteers showed consistently suppressed cortisol levels, confirming that the test remains reliable with repetition.
Key Clinical Pitfalls to Avoid
Don't rely on a single test: The guideline emphasizes that sensitivity of all screening tests exceeds 90%, but specificity is lower, with DST being among the least specific 1
Account for cyclic Cushing's: Multiple periodic sequential tests (especially LNSC) are particularly useful for detecting cyclic CS, where patients exhibit weeks to months of normal cortisol interspersed with excess episodes 1
Measure dexamethasone levels: This single intervention can substantially reduce false-positive results 1. In one study, all patients misclassified by testing had normal plasma dexamethasone concentrations 3
Don't test during disrupted sleep cycles: LNSC should not be performed in night-shift workers 1
Consider renal function: For patients with CrCl <60 mL/min or polyuria >5 L/24h, prefer LNSC over UFC 1
When Equivocal Results Persist
If screening tests remain indeterminate after proper technical verification and complementary testing, the combined LDDST-CRH test correctly diagnosed all indeterminate patients in one prospective series 3. However, the same study found that repeated assessment of screening tests with active follow-up achieved identical diagnostic accuracy.
The practical recommendation: Perform 2-3 measurements each of DST, LNSC, and UFC over time rather than relying on a single equivocal DST result. This approach aligns with guideline recommendations and accounts for both technical variability and the possibility of cyclic disease 1.