What are the precautions, contraindications, and risks of the dexamethasone (corticosteroid) suppression test in a 70-year-old patient suspected of having Cushing's syndrome?

Dexamethasone Suppression Test for Cushing's Syndrome in a 70-Year-Old Patient: Precautions, Contraindications, and Risks

The overnight 1-mg dexamethasone suppression test is generally safe for a 70-year-old patient with suspected Cushing's syndrome, but requires careful medication review, monitoring for false positives, and awareness of potential adrenal suppression effects. 1

Key Precautions

Medication Interactions

• Drug interactions: Several medications can interfere with test results:
  • CYP3A4 inducers (phenytoin, phenobarbital, ephedrine, rifampin) may enhance dexamethasone metabolism, causing false positives 1, 2
  • Carbamazepine can cause false positive results 3
  • Indomethacin can cause false negative results 2
  • Estrogens and oral contraceptives may increase corticosteroid binding globulin levels 2

Medical Conditions Affecting Results

• Renal function: Decreased glomerular filtration rate can lead to higher serum dexamethasone concentration 4
• Liver function: Patients with cirrhosis have enhanced corticosteroid effects 2
• Thyroid status: Hypothyroidism enhances corticosteroid effects 2
• Pseudo-Cushing's states: Psychiatric disorders, alcohol use disorder, and obesity can cause false positive results 1

Test Interpretation Considerations

• Dexamethasone levels: Measuring dexamethasone concomitantly with cortisol reduces risk of false positives 1, 4
• Method-specific cortisol cutoffs: Using appropriate laboratory-specific reference ranges improves accuracy 4

Contraindications

Absolute Contraindications

• Active psychosis or severe psychiatric disorders (may be worsened by dexamethasone) 2
• Active peptic ulcer disease (risk of exacerbation) 2

Relative Contraindications

• Poorly controlled diabetes (may worsen glycemic control) 2
• Poorly controlled hypertension 2
• Active infection, particularly fungal infections 2
• Osteoporosis (less relevant for single-dose test) 2

Risks to Discuss with the Patient

Short-term Risks (Single Dose)

• Mood changes: Potential for insomnia, mood swings, or euphoria even with short-term use 2
• Metabolic effects: Possible transient elevation in blood glucose 2
• False results: Risk of false positives or negatives due to medications or conditions 1, 2

Long-term Risks (Only for Repeated Testing)

• Adrenal suppression: Unlikely with single-dose testing but possible with repeated testing 2
• Steroid withdrawal syndrome: Fever, myalgia, arthralgia, and malaise may occur after prolonged therapy 2

Patient Preparation and Reassurance

Before the Test

1. Medication review: Document all medications and consider temporary discontinuation of interfering medications if possible 1, 2
2. Baseline measurements: Check blood pressure and blood glucose 2
3. Timing: The standard protocol involves taking 1 mg dexamethasone at 11 PM with blood cortisol measurement at 8 AM the next morning 1, 5

During the Test

1. Administration: Take dexamethasone exactly as prescribed (typically 1 mg orally at 11 PM) 1, 5
2. Monitoring: Be aware of any unusual symptoms (insomnia, mood changes) 2

After the Test

1. Follow-up: Arrange for appropriate follow-up testing if results are abnormal 1
2. Interpretation: Understand that a single abnormal result may require confirmation with additional tests 1, 6

Alternative Testing Options

If dexamethasone testing is contraindicated or problematic:

• Late-night salivary cortisol: Good option for patients with disrupted circadian rhythm or renal impairment 1
• 24-hour urinary free cortisol: Alternative screening test, though requires accurate 24-hour collection 1

By following these precautions and being aware of potential risks, the dexamethasone suppression test can be safely performed in a 70-year-old patient with suspected Cushing's syndrome, providing valuable diagnostic information with minimal risk.