What is the procedure for conducting a 1mg dexamethasone (corticosteroid) suppression test?

1mg Dexamethasone Suppression Test Procedure

The 1mg dexamethasone suppression test should be performed by administering 1mg of dexamethasone orally at 11:00 PM, followed by measurement of plasma cortisol between 8:00-9:00 AM the next morning, with a cortisol value <80 nmol/L (3 μg/dL) indicating normal suppression. 1, 2

Pre-Test Considerations

Before conducting the test, ensure the patient:

• Avoids eating, drinking, smoking, or brushing teeth 1-2 hours before collection
• Refrains from strenuous physical activity for 24-48 hours
• Avoids smoking/vaping/nicotine products for 4 hours
• Avoids alcohol consumption for 12 hours before sampling
• Has not had recent infections, especially respiratory tract infections within 2 weeks 1

Important Medication Considerations

• Women taking estrogen-containing oral contraceptives may have false positive results due to altered dexamethasone metabolism 1
• Document all current medications as many can interfere with test results

Test Protocol

1. Provide the patient with a 1mg dexamethasone tablet
2. Instruct the patient to take the tablet at 11:00 PM (note: research shows administration at 9:00 PM is equally effective) 3
3. The patient should return the following morning between 8:00-9:00 AM for blood collection
4. Collect blood sample for plasma cortisol measurement 2

Interpretation of Results

• Normal response: Morning cortisol suppresses to <80 nmol/L (3 μg/dL) 2
• Abnormal response: Morning cortisol ≥80 nmol/L (3 μg/dL) suggests possible Cushing's syndrome and requires further evaluation

Potential False Results

False Positives (failure to suppress despite absence of Cushing's syndrome)

• Severe obesity
• Uncontrolled diabetes
• Pregnancy
• Polycystic ovary syndrome (PCOS)
• Psychiatric disorders
• Alcohol use disorder 1

False Negatives (normal suppression despite Cushing's syndrome)

• Mild Cushing's syndrome
• Cyclic/periodic Cushing's syndrome 1, 4

Clinical Pearls

• The test has high specificity but limited sensitivity (as low as 18-41%) in patients with mild or episodic hypercortisolism 4
• In obese patients, the false positive rate is approximately 2.3% when using the 80 nmol/L cutoff 2
• If results are abnormal, confirm with additional tests such as 24-hour urinary free cortisol (UFC) or late-night salivary cortisol (LNSC) 1
• Some research suggests salivary cortisol measurement following dexamethasone may provide better sensitivity and specificity than plasma cortisol 5

Follow-Up Testing

If the test is abnormal, the Endocrine Society recommends a three-step approach:

1. Confirm hypercortisolism using additional tests (24-hour UFC, LNSC)
2. Determine if ACTH-dependent or independent by measuring plasma ACTH
3. Identify the source through appropriate imaging and dynamic testing 1