How do you interpret cortisol levels after dexamethasone (Decadron) suppression testing?

From the Guidelines

Interpretation of cortisol levels after dexamethasone suppression testing should be based on a cutoff value of 1.8 μg/dL (50 nmol/L), where levels below this threshold indicate a normal response and levels above suggest abnormal hypothalamic-pituitary-adrenal (HPA) axis function, potentially indicating Cushing's syndrome. This interpretation is guided by the most recent and highest quality study available, which emphasizes the importance of using a standardized protocol for the dexamethasone suppression test (DST) to ensure accurate results 1.

Key Considerations for Interpretation

• The timing of blood draws is critical, with cortisol levels typically measured between 8-9 AM following dexamethasone administration.
• Factors that can cause false positives, such as depression, alcoholism, obesity, and stress, or false negatives, such as medications affecting dexamethasone metabolism, should be considered when interpreting results.
• Measuring dexamethasone levels concomitantly with cortisol can help reduce the risk of false-positive results by accounting for variations in dexamethasone absorption and metabolism 1.

Protocol-Specific Interpretation

• For the standard overnight 1 mg dexamethasone suppression test, morning cortisol levels should be suppressed to below 1.8 μg/dL (50 nmol/L) in healthy individuals.
• The low-dose dexamethasone suppression test (0.5 mg every 6 hours for 48 hours) and the high-dose test (2 mg every 6 hours for 48 hours) have different interpretation criteria, with the high-dose test being useful for distinguishing between Cushing's disease and other forms of Cushing's syndrome 1.

Clinical Application

In clinical practice, the dexamethasone suppression test is a valuable tool for diagnosing and managing Cushing's syndrome. By understanding the expected physiological response to dexamethasone and considering potential factors that can influence test results, clinicians can accurately interpret cortisol levels and make informed decisions about patient care. The use of a standardized protocol and consideration of concomitant medications and patient factors are crucial for ensuring the reliability of test results 1.

From the FDA Drug Label

These interactions may interfere with dexamethasone suppression tests which should be interpreted with caution during administration of these drugs False negative results in the dexamethasone suppression test (DST) in patients being treated with indomethacin have been reported. Thus, results of the DST should be interpreted with caution in these patients

The interpretation of cortisol levels after dexamethasone suppression testing should be done with caution.

• Interpret results with caution during administration of certain drugs, such as phenytoin, phenobarbital, ephedrine, and rifampin, as they may enhance the metabolic clearance of corticosteroids.
• Be aware of potential false negative results in patients being treated with indomethacin. 2

From the Research

Interpreting Cortisol Levels after Dexamethasone Suppression Testing

To interpret cortisol levels after dexamethasone suppression testing, it's essential to consider the dose of dexamethasone used and the timing of cortisol measurement.

• The normal value after dexamethasone suppression is less than 2 micrograms/dL, rather than less than 5 micrograms/dL, as previously accepted 3.
• Simultaneous measurement of both plasma levels of dexamethasone and cortisol has proven useful in identifying patients with unsatisfactory dexamethasone suppression tests 4.
• The low-dose dexamethasone suppression test may yield false-negative results in some patients with Cushing's syndrome, and serum cortisol after the test can correlate with baseline urinary free cortisol, plasma ACTH level, and urinary free cortisol after the 2-day test 5.

Diagnostic Criteria for Cushing's Syndrome

The diagnostic criteria for Cushing's syndrome using the dexamethasone suppression test have been reevaluated in several studies:

• A study found that the overnight 8-mg dexamethasone suppression test had low specificity for the diagnosis of Cushing's disease when performed as originally described, but with revised sampling times and diagnostic criteria, the test had sensitivity and specificity similar to those of the conventional 6-day dexamethasone suppression test 6.
• Another study suggested that the Metyrapone test gave the safest classification in patients with Cushing's syndrome, while the Dexamethasone test reliably diagnosed Cushing's disease when suppression of serum cortisol was present following the large dose of Dexamethasone 7.

Considerations for Test Interpretation

When interpreting the results of dexamethasone suppression testing, consider the following:

• The dose of dexamethasone used: low-dose (1 mg) or high-dose (8 mg) 4, 6.
• The timing of cortisol measurement: morning after dexamethasone administration 3, 6.
• The correlation between serum cortisol and other parameters, such as urinary free cortisol and plasma ACTH level 5.