How to Interpret the Dexamethasone Suppression Test
A normal dexamethasone suppression test shows serum cortisol <1.8 μg/dL (50 nmol/L) at 8 AM after 1 mg dexamethasone given at 11 PM-midnight, which effectively rules out Cushing's syndrome. 1
Test Protocol and Normal Response
The overnight 1-mg DST is performed by:
- Administering 1 mg dexamethasone orally between 11 PM and midnight
- Measuring serum cortisol between 8-9 AM the following morning
- Normal response: cortisol <1.8 μg/dL (50 nmol/L) 1
In healthy individuals, this supraphysiologic dexamethasone dose suppresses ACTH and cortisol secretion through negative feedback. Failure to suppress indicates autonomous cortisol production 2.
Interpreting Abnormal Results
Cortisol Levels and Clinical Significance
Cortisol <1.8 μg/dL: Normal - effectively excludes Cushing's syndrome and dysregulated cortisol production from adrenal incidentalomas 1
Cortisol 1.8-5 μg/dL: Equivocal zone - requires additional testing. This range has the highest rate of false-positives, particularly in patients on oral contraceptives 1, 3. Consider measuring free cortisol or dexamethasone levels to clarify results 3.
Cortisol >5 μg/dL (138 nmol/L): Strongly suggests Cushing's syndrome or autonomous cortisol secretion from an adrenal incidentaloma 1
Confirming Abnormal Results
When you get an abnormal DST result, perform a second different screening test (late-night salivary cortisol or 24-hour urinary free cortisol) 2. Two concordantly positive tests confirm the diagnosis and warrant proceeding to determine the etiology of Cushing's syndrome 2.
Critical Causes of False-Positive Results
You must actively exclude these conditions before accepting an abnormal DST:
Medications affecting dexamethasone metabolism:
- CYP3A4 inducers (phenobarbital, carbamazepine, phenytoin, rifampin, St. John's wort) accelerate dexamethasone clearance → falsely elevated cortisol 1, 4
- Measure dexamethasone levels simultaneously with cortisol (should be ≥0.1 mcg/dL for adequate dosing) 3
Conditions increasing cortisol-binding globulin (CBG):
- Oral contraceptives/estrogen therapy - the most common cause of false-positives in the 1.8-5 μg/dL range 1, 3
- Pregnancy
- Chronic active hepatitis
- These elevate total cortisol but not free (bioactive) cortisol 1
Malabsorption issues:
- Rapid gut transit, chronic diarrhea, celiac disease can prevent adequate dexamethasone absorption 1
Pseudo-Cushing states:
- Psychiatric disorders (depression, anxiety)
- Alcohol use disorder
- Severe obesity
- Polycystic ovary syndrome
- These activate the HPA axis physiologically 1
Critical Causes of False-Negative Results
Medications inhibiting dexamethasone metabolism:
- Fluoxetine, cimetidine, diltiazem slow dexamethasone clearance → artificially higher dexamethasone levels → excessive suppression 1
- Indomethacin specifically causes false-negative DST results 4
Low protein states:
- Nephrotic syndrome, severe malnutrition decrease CBG/albumin → lower total cortisol despite normal free cortisol 1
Enhanced Interpretation Strategies
For Equivocal Results (1.8-5 μg/dL)
Measure simultaneous dexamethasone level - if <0.1 mcg/dL, inadequate dosing occurred; repeat test 3
Measure free cortisol - upper normal limit is 48 ng/dL (men and women not on OCP) or 79 ng/dL (women on OCP). This eliminates false-positives from CBG elevation 3
Consider dexamethasone-CRH test - perform 48-hour low-dose DST (0.5 mg every 6 hours), then give CRH 2 hours after last dexamethasone dose. Cortisol >1.4 μg/dL 15 minutes post-CRH suggests Cushing's disease 2
Alternative Testing Approach
The 48-hour low-dose DST (0.5 mg every 6 hours for 48 hours) has better specificity than the overnight test, with normal suppression defined as cortisol <1.8 μg/dL measured 6 hours after the last dose 2. Use this when the overnight test is equivocal or when higher specificity is needed.
Common Pitfalls to Avoid
- Don't use the outdated 5 μg/dL cutoff - this misses patients with mild Cushing's syndrome who partially suppress 1, 5
- Don't ignore medication history - always review CYP3A4 inducers/inhibitors before testing 1, 4
- Don't test during acute illness or stress - physiologic cortisol elevation causes false-positives 2
- Don't accept a single abnormal test - always confirm with a second different screening test 2
- Don't forget cyclic Cushing's - if clinical suspicion remains high despite negative tests, repeat testing during symptomatic periods 1