What are the implications of a DST (Dexamethasone Suppression Test) cortisol result of 2.2?

Interpreting a DST Cortisol Result of 2.2

A DST cortisol result of 2.2 μg/dL indicates adequate suppression and is consistent with normal hypothalamic-pituitary-adrenal axis function, ruling out Cushing's syndrome. 1, 2

Understanding DST Results and Interpretation

The overnight dexamethasone suppression test (DST) is a first-line screening tool for suspected Cushing's syndrome. The interpretation depends on the specific cutoff values:

• Values below 2.4 μg/dL (66 nmol/L) using LC-MS/MS methods indicate normal suppression 3
• Values over 5 μg/dL generally identify patients with dysregulated cortisol secretion 1
• A result of 2.2 μg/dL falls within the normal suppression range

Factors That Could Affect Result Interpretation

Several factors can influence DST results and should be considered:

• Medication interactions:

  • CYP3A4 inducers (phenobarbital, carbamazepine, St. John's wort) can cause false positives 1, 2
  • CYP3A4 inhibitors (fluoxetine, cimetidine, diltiazem) can cause false negatives 1

• Physiological factors:

  • Increased CBG levels from oral estrogens, pregnancy, or chronic active hepatitis can increase total cortisol levels 1, 2
  • Decreased CBG and albumin levels (as in nephrotic syndrome) might produce falsely low values 1

• Absorption issues:

  • Rapid absorption/malabsorption of dexamethasone due to increased gut transit time, chronic diarrhea, or celiac disease can affect results 1
  • Inadequate dexamethasone levels (<1.8 ng/mL) may lead to false positive results 3, 4

Next Steps Based on This Result

Since 2.2 μg/dL indicates adequate suppression:

1. No further testing for Cushing's syndrome is needed if the clinical suspicion was low to moderate

2. If clinical suspicion remains high despite this normal result:

   • Consider measuring dexamethasone levels to confirm adequate absorption 1, 3
   • Perform additional confirmatory tests such as:
     • Late-night salivary cortisol (LNSC) 2
     • 24-hour urinary free cortisol (UFC) measurements (at least 2-3 collections) 1, 2

3. If testing was performed for an adrenal incidentaloma:

   • This result rules out autonomous cortisol secretion 5
   • ACTH levels following overnight dexamethasone suppression can provide additional confirmation 5

Common Pitfalls to Avoid

• Don't rely solely on a single test result: The Endocrine Society recommends multiple tests for diagnosis of Cushing's syndrome 2
• Consider pseudo-Cushing's states: Psychiatric disorders, alcohol use disorder, polycystic ovary syndrome, and obesity can all cause mild HPA axis activation 1, 2
• Verify dexamethasone absorption: Measuring dexamethasone levels concomitantly with cortisol can reduce false-positive results 1, 3, 4
• Be aware of borderline results: For cortisol values between 3.5-4.9 μg/dL, measuring corticosterone may provide additional discrimination 6, 7

In summary, a DST cortisol result of 2.2 μg/dL indicates normal cortisol suppression and makes Cushing's syndrome unlikely in the absence of other compelling clinical or biochemical evidence.