What is the interpretation of a patient's dexamethasone suppression test (DST) results with a total cortisol level of 0.9 and a dexamethasone level of 353?

DST Results Interpretation: Normal Suppression Confirmed

Your patient's dexamethasone suppression test demonstrates normal cortisol suppression with adequate dexamethasone levels, effectively ruling out Cushing's syndrome and autonomous cortisol secretion. 1, 2

Test Result Analysis

Cortisol Level Assessment

• Post-DST cortisol of 0.9 μg/dL is well below the diagnostic threshold of 1.8 μg/dL (50 nmol/L) that defines normal suppression 1, 2
• This result is approximately 50% of the upper limit of normal, indicating robust suppression of the hypothalamic-pituitary-adrenal axis 1
• Values >5 μg/dL would indicate overt Cushing's syndrome, making your result clearly normal 1, 2

Dexamethasone Level Verification

• Dexamethasone level of 353 ng/mL (3.53 μg/dL) confirms adequate drug absorption and metabolism 1, 2
• The minimum threshold for adequate dexamethasone concentration is 1.8 ng/mL (0.018 μg/dL), and your patient's level is nearly 200-fold higher 2
• This eliminates false-negative results from rapid dexamethasone metabolism, CYP3A4 inducers, or malabsorption 1, 2
• Measuring dexamethasone levels concomitantly reduces false-positive DST results and confirms test validity 3, 1

Clinical Implications

Diagnostic Conclusions

• Cushing's syndrome is excluded with high confidence based on appropriate cortisol suppression with verified dexamethasone levels 1, 2
• The intact negative feedback mechanism of the HPA axis is demonstrated 2
• If this test was performed for adrenal incidentaloma evaluation, the mass can be classified as non-functioning from a cortisol perspective 1

No Further Testing Required

• No additional biochemical testing for hypercortisolism is warranted based on this normal result 1, 4
• Do not pursue invasive testing such as inferior petrosal sinus sampling, as IPSS should never be used to diagnose hypercortisolism 3, 4

Critical Pitfalls Avoided

Test Validity Confirmed

• The high dexamethasone level excludes common causes of false-positive DST results 1, 2:
  • CYP3A4 inducers (phenytoin, rifampin, carbamazepine) that accelerate dexamethasone metabolism
  • Malabsorption or inadequate dosing
  • Patient non-compliance with medication timing

Factors That Would Require Consideration (Not Applicable Here)

• Oral contraceptives/estrogen therapy can elevate cortisol-binding globulin, falsely increasing total cortisol, but this would cause elevated post-DST cortisol, not the low value seen here 1, 5
• Pseudo-Cushing's states (depression, alcoholism, severe obesity) can cause false-positive DST, but your patient's result is clearly negative 3, 1

Exception: Cyclic Cushing's Consideration

Only pursue further evaluation if clinical suspicion remains extremely high despite this normal result 3, 4:

• Cyclic Cushing's syndrome can produce weeks to months of normal cortisol secretion interspersed with hypercortisolism episodes 4
• This would require 3-6 months of serial testing (late-night salivary cortisol, 24-hour urinary free cortisol) during symptomatic periods 3, 4
• Dynamic testing and localization should be preceded by confirmatory testing to document active phase 3
• This approach is only justified with compelling clinical features of Cushing's syndrome 4