What is the purpose of a dexamethasone (corticosteroid) test?

From the Guidelines

The primary purpose of a dexamethasone test is to diagnose and differentiate conditions involving abnormal cortisol production, particularly Cushing's syndrome, by assessing the body's response to dexamethasone suppression. The dexamethasone suppression test is a crucial diagnostic tool in endocrinology, helping to identify abnormal cortisol regulation, which is essential for the proper diagnosis and treatment of endocrine disorders. In healthy individuals, dexamethasone suppresses cortisol production through negative feedback on the hypothalamic-pituitary-adrenal axis, whereas in Cushing's syndrome, cortisol levels remain elevated despite dexamethasone administration 1.

The most common version of this test is the overnight dexamethasone suppression test, where a patient takes dexamethasone (typically 1mg at 11pm) and then has their cortisol levels measured the following morning. A serum cortisol level of < 1.8 μg/dL (50 nmol/L) at 0800 h in the morning after 1 mg dexamethasone given between 2300 h and midnight is considered a normal response, strongly predicting the absence of Cushing's syndrome 1.

There are different versions of the dexamethasone suppression test, including the low-dose test (0.5mg every 6 hours for 48 hours) and the high-dose test (2mg every 6 hours for 48 hours), which help differentiate between various causes of Cushing's syndrome. The choice of test and interpretation of results should consider factors such as the patient's clinical history, medications, and potential interfering conditions, as outlined in recent guidelines 1.

It is crucial for patients to inform their doctor about any medications they are taking, as some can interfere with the test results. The dexamethasone test is valuable for identifying abnormal cortisol regulation, which is essential for the proper diagnosis and treatment of endocrine disorders, ultimately impacting morbidity, mortality, and quality of life. Recent studies and guidelines emphasize the importance of accurate diagnosis and management of Cushing's syndrome, highlighting the role of the dexamethasone suppression test in this process 1.

Key points to consider when interpreting the results of a dexamethasone test include:

• The cutoff values for normal and abnormal responses
• Potential interfering factors such as medications, renal impairment, and significant polyuria
• The need for repeated testing or additional diagnostic procedures to confirm the diagnosis
• The importance of clinical correlation and consideration of the patient's overall clinical picture.

By prioritizing the most recent and highest quality evidence, clinicians can ensure that patients receive accurate diagnoses and effective treatment plans, ultimately improving outcomes and quality of life 1.

From the FDA Drug Label

The FDA drug label does not answer the question.

From the Research

Purpose of the Dexamethasone Test

The dexamethasone test, also known as the dexamethasone suppression test, is used to diagnose and differentiate Cushing's syndrome, a condition caused by excess cortisol in the body. The test involves administering a dose of dexamethasone, a synthetic corticosteroid, and measuring the levels of cortisol in the blood or urine.

Types of Dexamethasone Tests

There are different types of dexamethasone tests, including:

• Low-dose dexamethasone suppression test: used to diagnose Cushing's syndrome 2, 3
• High-dose dexamethasone suppression test: used to differentiate between pituitary and ectopic causes of Cushing's syndrome 4, 5
• Overnight dexamethasone suppression test: used to diagnose Cushing's disease, a type of Cushing's syndrome caused by a pituitary tumor 6

Diagnostic Criteria

The diagnostic criteria for the dexamethasone test vary depending on the type of test and the laboratory. Generally, a suppression of cortisol levels to less than 50% of baseline is considered a positive result, indicating Cushing's syndrome. However, the criteria for normal suppression of cortisol after dexamethasone are controversial, and some studies suggest that low-dose dexamethasone may suppress cortisol levels to levels previously thought to exclude Cushing's syndrome 3.

Clinical Utility

The dexamethasone test is a useful tool in the diagnosis and differential diagnosis of Cushing's syndrome. The combination of the high-dose dexamethasone suppression test and the CRF test, with measurement of serum cortisol, is superior to either test alone in the differential diagnosis of Cushing's syndrome 5. The low-dose dexamethasone suppression test is highly sensitive in diagnosing Cushing's syndrome and, combined with the results of the serum cortisol response to the CRH test, offers a safe and cost-effective test in the differential diagnosis of ACTH-dependent Cushing's syndrome 2.

Key Findings

• The high-dose dexamethasone suppression test is a mainstay in the differential diagnosis of Cushing's syndrome 4
• The overnight dexamethasone suppression test has a sensitivity of 78% and a specificity of 100% for the diagnosis of Cushing's disease 6
• The low-dose dexamethasone suppression test is highly sensitive in diagnosing Cushing's syndrome, but may suppress cortisol levels to levels previously thought to exclude Cushing's syndrome 3
• The combination of the high-dose dexamethasone suppression test and the CRF test is superior to either test alone in the differential diagnosis of Cushing's syndrome 5