From the Guidelines
The Dexamethasone Suppression Test (DST) results are interpreted as follows: a serum cortisol level of less than 1.8 μg/dL after a 1-mg dexamethasone dose is considered a normal response, indicating the absence of Cushing's Syndrome (CS) 1.
Key Interpretation Points
- A cortisol level < 1.8 μg/dL excludes dysregulated cortisol production from an adrenal incidentaloma 1
- A cortisol level > 5 μg/dL generally identifies patients with dysregulated cortisol secretion from an incidentaloma with overt CS 1
- False positive results may occur due to:
- Rapid absorption/malabsorption of dexamethasone
- Concomitant treatment with CYP3A4 inducers (e.g., phenobarbital, carbamazepine, St. John’s wort)
- Increased corticosteroid binding globulin (CBG) levels from oral estrogens, pregnancy, or chronic active hepatitis 1
- False negative results may occur due to:
- Inhibition of dexamethasone metabolism by concomitant medications (e.g., fluoxetine, cimetidine, diltiazem)
- Decreased CBG and albumin levels (e.g., in patients with nephrotic syndrome) 1
Important Considerations
- Measuring dexamethasone concomitantly with cortisol can reduce the risk of false-positive results 1
- Using laboratory-specific ranges of expected values can also help reduce the risk of false-positive 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 Dexamethasone suppression test results should be done with caution in patients taking certain medications, such as:
- Phenytoin, phenobarbital, ephedrine, and rifampin, as they may enhance the metabolic clearance of corticosteroids
- Indomethacin, as it may cause false negative results Results of the DST should be interpreted with caution in these patients 2
From the Research
Interpretation of Dexamethasone Suppression Test Results
The interpretation of dexamethasone suppression test (DST) results can be complex and influenced by various factors.
- The test is used to diagnose Cushing's syndrome, and the results can be affected by dexamethasone absorption and metabolism, as well as altered cortisol binding 3.
- A study found that quantification of serum dexamethasone in DST samples can improve test performance and reduce false positive results 4.
- The use of method-specific cortisol cutoffs in DST samples can also increase clinical specificity for Cushing's syndrome while preserving clinical sensitivity 4.
- Another study found that low-dose dexamethasone suppression testing can yield false-negative results in some patients, and that serum cortisol after the test can correlate with baseline urinary free cortisol and plasma ACTH level 5.
- The interpretation of DST results can be enhanced with the use of synchronous free cortisol assessment, particularly in patients with optimal dexamethasone concentrations and post-DST total cortisol between 1.8 and 5 mcg/dL 3.
- A comparison of the standard high-dose dexamethasone suppression test and the overnight 8-mg dexamethasone suppression test found that the revised overnight test has sensitivity and specificity similar to those of the conventional 6-day test 6.
- The use of the DST in clinical practice requires consideration of the clinical context and technical factors, as well as common sense and attention to detail 7.
Factors Influencing DST Results
Several factors can influence DST results, including:
- Dexamethasone intake and serum exposure 4
- Cortisol binding and metabolism 3
- Patient characteristics, such as body mass index, sex, age, and use of oral contraceptives 4, 3
- Technical factors, such as the use of liquid chromatography-tandem mass spectrometry for quantification of dexamethasone and cortisol 4
Clinical Applications
The DST has several clinical applications, including: