What is the procedure for the dexamethasone (corticosteroid) suppression test?

From the Guidelines

The dexamethasone suppression test should be performed by administering 1 mg of dexamethasone orally between 2300 h and midnight, and then measuring serum cortisol levels at 0800 h the next morning, with a level < 1.8 μg/dL (50 nmol/L) considered a normal response 1. To perform the test, first collect a baseline blood sample for cortisol measurement in the morning. Then administer oral dexamethasone, typically 1 mg for the standard overnight test. For the overnight test, give dexamethasone at 11 PM and collect a second blood sample for cortisol measurement at 8 AM the following morning. Some key considerations for the test include:

• Measuring dexamethasone levels along with cortisol can improve test interpretability 1
• Patients should avoid medications that affect dexamethasone metabolism, such as phenytoin, phenobarbital, or rifampin, for at least 24 hours before and during the test 1
• The test should be performed under medical supervision to ensure proper interpretation of results
• False positive results may be seen with rapid absorption/malabsorption of dexamethasone, concomitant treatment with CYP3A4 inducers, or increased corticosteroid binding globulin (CBG) levels 1
• False negative results are less common, but may result from inhibition of dexamethasone metabolism by concomitant medications or decreased CBG and albumin levels 1.

From the Research

Dexamethasone Suppression Test Procedure

The dexamethasone suppression test (DST) is a laboratory test used to diagnose and differentiate various endocrine disorders, including Cushing's syndrome and depression. The test involves administering a dose of dexamethasone, a synthetic steroid, and measuring the levels of cortisol in the blood or urine after a certain period.

Types of Dexamethasone Suppression Tests

There are different types of DSTs, including:

• Low-dose test: used for screening and initial assessment of hypercortisolemia 2
• High-dose test: used for more elaborate diagnostics, but its clinical value is questionable 2
• Overnight dexamethasone suppression test: used to screen for Cushing's syndrome, but has limitations in patients with mild and/or episodic hypercortisolism 3
• Very high dose dexamethasone suppression test: used for differential diagnosis of Cushing's syndrome, particularly in patients with pituitary Cushing's disease 4

Factors Affecting Dexamethasone Suppression Test Results

Several factors can affect the results of the DST, including:

• Dexamethasone absorption and metabolism 5
• Altered cortisol binding 5
• Use of oral contraceptive therapy (OCT) 5
• Organ dysfunction 2
• Pseudo-Cushing states 2
• Pregnancy and other physiological conditions leading to elevated blood cortisol 2
• Cyclic Cushing disease 2

Interpretation of Dexamethasone Suppression Test Results

The interpretation of DST results can be enhanced by using synchronous free cortisol assessment 5. The normal ranges of free cortisol during DST in participants without adrenal disorders have been established, and post-DST free cortisol measurements can add value to the diagnostic workup in certain patients 5.

Key Considerations

• The DST is not suitable for screening unselected patients, but should be used when the clinician thinks it might help to answer a specific diagnostic question 6
• The test results should always be interpreted in clinical context, and physicians should treat patients, not laboratory results 6
• The use of DSTs requires careful consideration of the potential factors that can affect the results, and the selection of the appropriate type of test for the specific clinical scenario 2, 4, 3, 5