What are the diagnostic tests for Cushing's disease?

Diagnostic Testing for Cushing's Disease

The diagnostic approach for Cushing's disease involves a two-step process: first confirming hypercortisolism with screening tests (24-hour urinary free cortisol, late-night salivary cortisol, or 1mg overnight dexamethasone suppression test), then determining the source through ACTH measurement and appropriate imaging studies. 1

Initial Screening Tests for Hypercortisolism

The Endocrine Society recommends using one of the following first-line tests to screen for Cushing's syndrome in patients with suspicious clinical features (unexplained weight gain, hypertension, glucose intolerance):

1. 24-hour Urinary Free Cortisol (UFC)

   • Reflects integrated tissue exposure to free cortisol over 24 hours
   • Sensitivity: 97%, Specificity: 91% 2
   • Requires complete 24-hour urine collection
   • At least 2-3 collections recommended for confirmation 1
   • Most accurate when measured by liquid chromatography tandem-mass spectrometry 2

2. Late-Night Salivary Cortisol (LNSC)

   • Sensitivity: >90%, Specificity: 100% 1
   • Measures disruption of normal cortisol circadian rhythm
   • Particularly useful for cyclic Cushing's syndrome surveillance
   • Simple collection method suitable for outpatients 3
   • Collection device can be mailed to laboratory 3

3. 1mg Overnight Dexamethasone Suppression Test (DST)

   • Morning serum cortisol <50 nmol/L generally excludes Cushing's syndrome 3
   • Recommended for initial screening, adrenal incidentalomas, and patients with disrupted circadian rhythms 1
   • A cutoff of 138 nmol/L offers best specificity (97%) 2

Important caveat: In mild Cushing's syndrome, a single normal test does not exclude the diagnosis. Multiple samples and tests may be needed for diagnosis 4.

Determining ACTH Status and Source Localization

After confirming hypercortisolism, the next step is to determine whether the condition is ACTH-dependent or ACTH-independent:

1. Plasma ACTH Measurement

   • Differentiates between ACTH-dependent (pituitary or ectopic source) and ACTH-independent (adrenal source) Cushing's syndrome 1
   • Essential for directing subsequent imaging studies

2. Imaging Studies

   • Pituitary MRI: For suspected Cushing's disease (pituitary origin)
   • Adrenal CT/MRI: For suspected adrenal source
   • Chest/Abdominal Imaging: For suspected ectopic ACTH-secreting tumors

3. Bilateral Inferior Petrosal Sinus Sampling (BIPSS)

   • Used when imaging is inconclusive
   • Helps identify the source of excess cortisol production 1
   • Particularly valuable when pituitary MRI is normal but Cushing's disease is suspected 4

Potential Pitfalls and Considerations

• False positives in UFC: Incomplete collections, high fluid intake, or certain medications can affect results
• False positives in DST: Estrogen therapy, pregnancy (increased CBG levels), certain medications that affect CYP3A4 1
• False negatives in all tests: Mild or cyclic Cushing's syndrome may be missed with single measurements 4
• Laboratory methods: Antibody-based immunoassays may generate false positives due to cross-reactivity; structurally-based assays like liquid chromatography with tandem mass spectrometry are more accurate 3
• Hepatic impairment: Patients with cirrhosis may have an impaired response to tests using metyrapone 5

Testing Algorithm

1. Initial screening: Choose at least one of the three first-line tests (UFC, LNSC, or DST)
2. If positive: Confirm with a second different test
3. If confirmed: Measure plasma ACTH to determine if ACTH-dependent or independent
4. Based on ACTH status: Order appropriate imaging studies
5. If imaging inconclusive: Consider BIPSS for suspected Cushing's disease with normal pituitary MRI

For optimal diagnostic accuracy, combining multiple tests increases sensitivity and specificity, though UFC measured by liquid chromatography tandem-mass spectrometry has been shown to achieve the best accuracy among individual tests 2.