What is the best test to confirm Cushing's syndrome?

Confirming Cushing's Syndrome: Diagnostic Testing

Initial Screening Tests

To confirm Cushing's syndrome, use at least two of three first-line screening tests: 24-hour urinary free cortisol (UFC), late-night salivary cortisol (LNSC), or overnight 1-mg dexamethasone suppression test (DST), with UFC and LNSC being preferred for their high sensitivity (>90%) and specificity (approaching 100%). 1, 2

The diagnostic approach requires:

• 24-hour urinary free cortisol (UFC): Collect for 3 consecutive days with diagnostic cut-off >193 nmol/24h (>70 μg/m²), demonstrating 89% sensitivity and 100% specificity 3, 1
• Late-night salivary cortisol (LNSC): Shows 92-95% sensitivity and 100% specificity, making it the most useful single screening test 2, 4
• Overnight 1-mg dexamethasone suppression test: Serum cortisol >1.8 μg/dL (50 nmol/L) at 8 AM after 1 mg dexamethasone given at midnight indicates Cushing's syndrome 3, 1

Critical Diagnostic Principles

Perform at least 2-3 abnormal screening tests before confirming the diagnosis, as no single test achieves 100% diagnostic accuracy 3, 2. This is particularly important because:

• Mild or cyclic Cushing's syndrome may show normal results on single collections 5, 6
• Neither a normal UFC nor normal LNSC excludes mild Cushing's syndrome 5
• Multiple samples increase diagnostic yield significantly 5, 7

Test Selection Strategy

Start with UFC and/or LNSC as first-line tests, choosing based on patient-specific factors 1:

• Prefer UFC when: General screening is needed and patient can reliably collect 24-hour urine samples 1
• Prefer LNSC when: Patient has renal impairment (CrCl <60 mL/min), polyuria (>5 L/24h), or compliance concerns with urine collection 1, 2
• Avoid LNSC in: Night-shift workers due to disrupted circadian rhythm 2

Confirming Hypercortisolism in Children and Adolescents

For pediatric patients, the diagnostic protocol includes 3:

• UFC excretion for 3 days: >193 nmol/24h (>70 μg/m²) with 89% sensitivity and 100% specificity 3
• Serum cortisol circadian rhythm: Midnight sleeping cortisol ≥50 nmol/L (≥1.8 μg/dL) with 100% sensitivity and 60% specificity 3
• Late-night salivary cortisol: Based on local assay cut-off with 95% sensitivity and 100% specificity 3
• Low-dose dexamethasone suppression test (LDDST): 0.5 mg 6-hourly for 48 hours, with serum cortisol >50 nmol/L (>1.8 μg/dL) indicating Cushing's syndrome 3

Common Pitfalls to Avoid

Never rely on a single UFC collection or use ACTH stimulation testing to diagnose Cushing's syndrome 1. Key considerations include:

• Exclude exogenous glucocorticoid use before any biochemical testing 3, 1
• Ensure complete 24-hour urine collections: Incomplete collections yield false-negative results 2
• Measure dexamethasone levels along with cortisol during DST to rule out abnormal dexamethasone metabolism 1, 2
• Recognize false positives occur with severe obesity, uncontrolled diabetes, depression, alcoholism, pregnancy, CYP3A4 inducers, and oral estrogens 1, 2

Next Steps After Confirming Hypercortisolism

Once hypercortisolism is confirmed with abnormal screening tests:

• Measure morning (09:00h) plasma ACTH to determine if Cushing's syndrome is ACTH-dependent (ACTH >5 ng/L) or ACTH-independent (low/undetectable ACTH) 3, 1, 8
• For ACTH-dependent disease: Proceed to pituitary MRI to identify potential pituitary adenoma 1, 8
• For ACTH-independent disease: Perform adrenal CT or MRI to identify adrenal lesion(s) 1, 8

Special Considerations for Mild or Cyclic Disease

In patients with high clinical suspicion but equivocal initial testing, repeat screening tests multiple times 5. The diagnostic approach should include:

• Multiple UFC collections (not just one) to capture episodic hypercortisolism 5, 7
• Serial LNSC measurements, with 14-100% of samples potentially elevated in mild cases 5
• Dex-CRH test for distinguishing true Cushing's syndrome from pseudo-Cushing's states, with 90% sensitivity and 95% specificity 1