When to administer dexamethasone (Decadron) for adrenal suppression test?

From the Guidelines

Dexamethasone should be given for a suppression test when evaluating for conditions involving excess cortisol production, particularly Cushing's syndrome, with the 1 mg overnight test being the preferred screening method, administered at 11 PM or midnight, and blood cortisol measured the following morning between 8-9 AM, as recommended by the most recent guidelines 1.

Key Considerations

• The low-dose dexamethasone suppression test is used to diagnose and differentiate between various forms of Cushing's syndrome, including pituitary-dependent Cushing's disease, ectopic ACTH production, and adrenal causes of hypercortisolism 1.
• The test involves administering dexamethasone, which suppresses ACTH production in the pituitary, and subsequently, cortisol production in normal individuals, but may not suppress cortisol production in individuals with Cushing's syndrome 1.
• Patients should avoid medications that affect dexamethasone metabolism, such as phenytoin, phenobarbital, or rifampin, for at least one week before testing, and estrogen-containing medications should be discontinued for 6 weeks prior to testing to prevent false positive results 1.

Test Administration

• The 1 mg overnight dexamethasone suppression test is the preferred screening method, with dexamethasone administered at 11 PM or midnight, and blood cortisol measured the following morning between 8-9 AM 1.
• The standard two-day low-dose test involves administering dexamethasone 0.5 mg every 6 hours for 48 hours, with cortisol measurements at the end, while the high-dose test uses 2 mg every 6 hours for 48 hours or a single 8 mg dose at 11 PM 1.

Interpretation

• A serum cortisol level < 1.8 μg/dL (50 nmol/L) at 0800 h in the morning after 1 mg dexamethasone administration is considered a normal response, while a level > 5 μg/dL (138 nmol/L) may indicate dysregulated cortisol secretion 1.
• Measuring dexamethasone levels concomitantly with cortisol can improve test interpretability and reduce the risk of false-positive results 1.

From the Research

Dexamethasone Suppression Test Timing

The dexamethasone suppression test is used to diagnose Cushing's Syndrome (CS) and other disorders related to hypercortisolism. The test involves administering a low dose of dexamethasone, a synthetic glucocorticoid, to suppress cortisol production.

• The test is typically performed overnight, with dexamethasone administered at 11 pm and cortisol levels measured the next morning 2, 3.
• Some studies have used a 14-day course of prednisone followed by a dexamethasone suppression test to predict the development of impaired adrenal function 4.
• The timing of the test may vary depending on the specific protocol and the patient's condition.

Factors Affecting Test Results

Several factors can affect the results of the dexamethasone suppression test, including:

• Dexamethasone bioavailability and metabolism, which can be influenced by various substances and medications 5.
• Cortisol binding globulin production, which can be enhanced by certain drugs 5.
• Organ dysfunction, pseudo-Cushing states, pregnancy, and other physiological conditions that can lead to elevated blood cortisol levels 5.
• The use of oral contraceptive therapy (OCP), which can affect cortisol levels and test results 6.

Interpretation of Test Results

The interpretation of dexamethasone suppression test results can be enhanced by:

• Measuring serum dexamethasone levels to ensure adequate suppression of cortisol production 2, 3.
• Using synchronous free cortisol assessment to improve diagnostic accuracy 6.
• Considering the patient's clinical presentation, medical history, and other diagnostic tests to confirm or rule out CS or other disorders related to hypercortisolism 2, 3.