What are the initial laboratory evaluations for suspected Cushing's syndrome?

Initial Laboratory Evaluations for Suspected Cushing's Syndrome

The initial laboratory evaluations for suspected Cushing's syndrome should include late-night salivary cortisol, 24-hour urinary free cortisol, and overnight 1 mg dexamethasone suppression test, with at least two different tests performed to maximize diagnostic accuracy. 1

Diagnostic Approach

Step 1: Rule Out Exogenous Glucocorticoid Use

• Determine if patient takes any form of exogenous glucocorticoids (oral, injections, inhalers, topical)
• If yes, stop glucocorticoids if possible before proceeding with testing 1

Step 2: Initial Screening Tests

Based on clinical suspicion level:

Low Clinical Suspicion:

• Single screening test (choose one):
  • Late-night salivary cortisol (LNSC)
  • 24-hour urinary free cortisol (UFC)
  • Overnight 1 mg dexamethasone suppression test (DST)

Intermediate to High Clinical Suspicion:

• Perform 2-3 screening tests 1
  • LNSC (≥2 samples on consecutive days)
  • 24-hour UFC (2-3 collections)
  • Overnight 1 mg DST

Step 3: Interpretation of Results

• If normal: Cushing's syndrome unlikely
• If abnormal: Repeat 1-2 screening tests to confirm
• If results remain abnormal: Proceed to determine etiology
• If results are discordant: Consider cyclic Cushing's syndrome or repeat testing 1

Test Characteristics and Performance

Late-Night Salivary Cortisol (LNSC)

• Sensitivity: 95%
• Specificity: 100%
• Collection time: 22:00-23:00h
• Advantages:
  • Non-invasive, easy collection at home
  • Reflects free (active) cortisol
  • Multiple samples can be easily collected 1, 2
• Limitations:
  • Lower specificity in adrenal tumors 1
  • May miss mild cases of Cushing's syndrome 3

24-Hour Urinary Free Cortisol (UFC)

• Sensitivity: 89%
• Specificity: 100%
• Diagnostic cut-off: >193 nmol/24h (>70 μg/m²)
• Advantages:
  • Integrates cortisol production over 24 hours
  • Not affected by cortisol-binding globulin levels 1
• Limitations:
  • Inconvenient collection
  • May be normal in mild cases 3
  • Requires complete collection

Overnight 1 mg Dexamethasone Suppression Test (DST)

• Sensitivity: 95%
• Specificity: 80%
• Protocol:
  • Adults: 1 mg dexamethasone at 23:00h, measure serum cortisol at 09:00h next day
  • Children: 25 μg/kg (maximum 1 mg) at 23:00h
• Diagnostic cut-off: ≥50 nmol/L (≥1.8 μg/dL) indicates failure to suppress
• Advantages:
  • Simple to perform
  • Good first-line test for adrenal tumors 1
• Limitations:
  • False positives with estrogen therapy
  • False negatives in up to 18% of Cushing's disease patients 4
  • Measuring dexamethasone levels improves interpretation 1

Important Caveats and Pitfalls

Conditions That May Cause False Positive Results

• Severe obesity
• Uncontrolled diabetes mellitus
• Pregnancy
• Polycystic ovary syndrome
• Depression
• Alcoholism
• Physical stress (illness, surgery)
• Excessive exercise 1

Test Selection Considerations

• For shift workers: DST may be preferable to LNSC
• For women on estrogen-containing contraceptives: Avoid DST
• For suspected adrenal tumor: Start with DST 1
• For suspected mild Cushing's syndrome: Multiple samples needed (UFC/LNSC) 3

Critical Points

1. No single test has 100% diagnostic accuracy; multiple tests increase sensitivity
2. Normal results on a single test do not exclude mild Cushing's syndrome
3. Cyclic Cushing's syndrome may require periodic re-evaluation
4. Consider measuring dexamethasone levels along with cortisol during DST to ensure adequate absorption 1

Next Steps After Positive Screening

If screening tests confirm hypercortisolism:

1. Measure plasma ACTH to differentiate ACTH-dependent from ACTH-independent causes

   • ACTH-dependent: ACTH normal or elevated (>5 ng/L or >1.1 pmol/L)
   • ACTH-independent: ACTH low or undetectable 1

2. Proceed with appropriate imaging based on ACTH results:

   • ACTH-dependent: Pituitary MRI
   • ACTH-independent: Adrenal CT or MRI 1

By following this systematic approach to laboratory evaluation, clinicians can effectively diagnose Cushing's syndrome while minimizing false results, ultimately improving patient outcomes by enabling earlier treatment.