From the Guidelines
The dexamethasone suppression test protocol involves administering 1 mg of oral dexamethasone at 11 PM, followed by a blood draw for cortisol measurement between 8-9 AM the next morning, with normal results showing cortisol suppression to less than 1.8 μg/dL (50 nmol/L) 1.
Key Considerations
- The test is used to assess the hypothalamic-pituitary-adrenal axis function and diagnose conditions like Cushing's syndrome.
- Patients should avoid alcohol, stress, and medications that affect dexamethasone metabolism for at least one week before testing.
- The test works because dexamethasone normally suppresses ACTH production from the pituitary, which reduces cortisol secretion.
- Failure to suppress cortisol suggests conditions like Cushing's syndrome, depression, or alcoholism.
Protocol Details
- For the standard overnight test, the patient takes 1 mg of oral dexamethasone at 11 PM.
- A blood draw for cortisol measurement is taken between 8-9 AM the next morning.
- Normal results show cortisol suppression to less than 1.8 μg/dL (50 nmol/L) 1.
- Measuring dexamethasone concomitantly with cortisol can reduce the risk of false-positive results 1.
Special Considerations
- 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, typically resulting from inhibition of dexamethasone metabolism by concomitant medications or decreased CBG and albumin levels 1.
Recent Guidelines
- The most recent guidelines recommend using the overnight 1-mg dexamethasone suppression test as a diagnostic tool for Cushing's syndrome 1.
- The guidelines also suggest measuring dexamethasone levels along with cortisol to improve test interpretability 1.
From the Research
Dexamethasone Suppression Test Protocol
The dexamethasone suppression test is a widely used screening tool for Cushing's syndrome, a condition characterized by excess cortisol production. The test protocol typically involves administering a low dose of dexamethasone, a synthetic corticosteroid, and measuring the subsequent cortisol levels in the blood or urine.
Test Procedure
- The overnight 1-mg dexamethasone suppression test involves giving 1 mg of dexamethasone at midnight, and then measuring the plasma cortisol level at 8:00 the following morning 2.
- The test can also be performed over a longer period, such as 24 or 48 hours, with multiple measurements of cortisol levels 3.
- In some cases, a combined dexamethasone-CRH (corticotropin-releasing hormone) test may be used to distinguish Cushing's syndrome from pseudo-Cushing's states 4.
Interpretation of Results
- A cortisol level above a certain threshold (e.g., 5 microg/dL) after dexamethasone administration may indicate Cushing's syndrome 2.
- However, the sensitivity and specificity of the test can vary depending on the population being tested and the criteria used to define a positive result 2, 3.
- The test may not be reliable in patients with mild or episodic Cushing's syndrome, as they may suppress cortisol production in response to dexamethasone 2.
Factors Affecting Test Results
- Obesity is not a confounding factor in the 1-mg dexamethasone suppression test, as obese individuals suppress cortisol production similarly to non-obese individuals 5.
- The dexamethasone-suppressed CRH test can differentiate mild Cushing's disease from normal physiology, but its value in patients with episodic hormonogenesis has not been tested 6.