What is the dexamethasone (corticosteroid) suppression test?

From the Guidelines

The dexamethasone suppression test is a crucial diagnostic tool for evaluating the function of the hypothalamic-pituitary-adrenal (HPA) axis, particularly in screening for Cushing's syndrome, where a serum cortisol level < 1.8 μg/dL at 0800 h after 1 mg dexamethasone administration indicates a normal response 1. The test procedure typically involves administering 1 mg of dexamethasone orally between 2300 h and midnight, followed by a blood cortisol measurement at 0800 h the next morning.

• Key aspects of the test include:
  • In healthy individuals, dexamethasone inhibits vasopressin and ACTH secretion, leading to decreased cortisol levels.
  • A serum cortisol level < 1.8 μg/dL (50 nmol/L) after dexamethasone administration is considered a normal response, strongly predicting the absence of Cushing's syndrome.
  • False-positive results can occur due to factors such as rapid absorption or malabsorption of dexamethasone, concomitant treatment with CYP3A4 inducers, and increased corticosteroid binding globulin (CBG) levels.
  • Measuring dexamethasone concomitantly with cortisol can help reduce the risk of false-positive results.
• It is essential to consider potential factors that may affect test results, such as concomitant medications and certain medical conditions, to ensure accurate interpretation of the dexamethasone suppression test results, as highlighted in the guideline update on the diagnosis and management of Cushing's disease 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 dexamethasone suppression test (DST) is a test that may be affected by the administration of certain drugs, including those that enhance the metabolic clearance of corticosteroids, such as phenytoin, phenobarbital, ephedrine, and rifampin 2.

• The test may produce false negative results in patients being treated with indomethacin.
• The results of the DST should be interpreted with caution during administration of these drugs.

From the Research

Definition and Purpose of the Dexamethasone Suppression Test

• The dexamethasone suppression test (DST) is a diagnostic tool used to assess the functioning of the adrenal glands and to diagnose Cushing's syndrome 3, 4, 5.
• The test involves administering a dose of dexamethasone, a synthetic glucocorticoid, and then measuring the levels of cortisol in the blood or urine to see if they are suppressed 3, 4.
• The purpose of the test is to determine if the adrenal glands are producing excessive amounts of cortisol, which can indicate Cushing's syndrome or other adrenal disorders 5.

Types of Dexamethasone Suppression Tests

• There are several types of DST, including the low-dose and high-dose tests, which can be administered orally or intravenously 3, 6.
• The low-dose test is used to diagnose Cushing's syndrome, while the high-dose test is used to distinguish between different causes of the syndrome 3, 6.
• The test can also be performed as a short outpatient procedure, making it more convenient and less cumbersome than traditional testing methods 3.

Interpretation of Test Results

• The results of the DST are interpreted based on the levels of cortisol in the blood or urine after administration of dexamethasone 4, 7.
• A decrease in cortisol levels indicates that the adrenal glands are responding normally to the dexamethasone, while an increase or no change in cortisol levels can indicate Cushing's syndrome or other adrenal disorders 4, 7.
• The test can also be used to monitor the effectiveness of treatment for Cushing's syndrome and to adjust medication dosages as needed 5, 6.

Clinical Utility and Limitations

• The DST is a useful diagnostic tool for Cushing's syndrome, but it is not without limitations 3, 4.
• The test can produce false positive results if the patient has not taken the dexamethasone as directed or if there are other factors affecting cortisol levels 4.
• The test can also be affected by other medical conditions, such as renal or hepatic impairment, and by certain medications 4, 5.
• Therefore, the results of the DST should be interpreted in conjunction with other diagnostic tests and clinical evaluations to ensure accurate diagnosis and treatment of Cushing's syndrome 5, 6.