Interpretation of Dexamethasone Suppression Test Results
This result indicates normal cortisol suppression and effectively rules out Cushing's syndrome. The cortisol level of 1.1 μg/dL is well below the diagnostic threshold of 1.8 μg/dL, and the dexamethasone level of 426 ng/mL confirms adequate drug absorption and test validity 1, 2.
Understanding the Test Results
Your patient has demonstrated intact hypothalamic-pituitary-adrenal axis function with appropriate negative feedback. The cortisol value of 1.1 μg/dL after overnight dexamethasone suppression is substantially below the normal cutoff of <1.8 μg/dL (50 nmol/L), indicating proper suppression 1, 2, 3.
Dexamethasone Level Significance
The dexamethasone level of 426 ng/mL is critical for interpreting this result:
- This elevated dexamethasone level confirms the test's validity by demonstrating adequate drug absorption and ruling out false-negative results from rapid metabolism or malabsorption 1, 2, 3
- The lower limit of normal for dexamethasone is 1.8 ng/mL (4.6 nmol/L), so a level of 426 ng/mL far exceeds this threshold 3
- Measuring dexamethasone concomitantly with cortisol reduces false-positive DST results and is considered best practice 1, 2
Clinical Implications
No further biochemical testing for hypercortisolism is indicated based on this result. The combination of suppressed cortisol and adequate dexamethasone levels provides high confidence in excluding autonomous cortisol secretion 1, 3.
Specific Next Steps
- If this test was performed for adrenal incidentaloma workup, the mass can be classified as non-functioning from a cortisol perspective 1
- If evaluating for Cushing's syndrome, this result excludes the diagnosis with high confidence 1, 3
- No additional screening tests (24-hour urinary free cortisol, late-night salivary cortisol) are needed unless clinical suspicion remains extremely high for cyclic Cushing's syndrome 1, 2
Critical Pitfalls to Avoid
When to Reconsider Testing
Only pursue additional evaluation if:
- Clinical features of Cushing's syndrome are severe and progressive despite this normal result 2
- There is strong suspicion for cyclic Cushing's syndrome, which can produce weeks to months of normal cortisol secretion interspersed with hypercortisolism 1, 2
- In cyclic disease, repeat testing during symptomatic periods over 3-6 months may be warranted 2
Understanding Test Limitations
While guidelines recommend a cutoff of 1.8 μg/dL, research demonstrates that some patients with mild or episodic Cushing's syndrome can suppress below this threshold 4, 5. However, with a cortisol of 1.1 μg/dL and confirmed adequate dexamethasone levels, the likelihood of missed disease is extremely low 1, 3.
The high dexamethasone level in your patient eliminates concerns about false-negative results from CYP3A4 inducers (phenobarbital, carbamazepine, St. John's wort) that accelerate dexamethasone metabolism 2, 3.
Documentation and Follow-Up
Document that the patient has:
- Normal cortisol suppression (1.1 μg/dL, well below 1.8 μg/dL threshold) 1, 2
- Adequate dexamethasone absorption confirmed (426 ng/mL) 1, 3
- Intact HPA axis negative feedback 3
- No evidence of autonomous cortisol secretion 1
If the patient has an adrenal incidentaloma, proceed with evaluation for pheochromocytoma and primary aldosteronism as clinically indicated, but cortisol excess is excluded 3.