What are the recommended diagnostic tests for Cushing's disease?

Diagnostic Testing for Cushing's Disease

The recommended diagnostic approach for Cushing's disease begins with initial screening tests including 24-hour urinary free cortisol (UFC), late-night salivary cortisol (LNSC), and/or the overnight 1-mg dexamethasone suppression test (DST), followed by ACTH measurement to determine if the condition is ACTH-dependent or independent. 1, 2

Initial Screening Tests

First-line Tests (choose at least one):

1. Late-Night Salivary Cortisol (LNSC)

   • Advantages: Easy collection, stable at room temperature, useful for longitudinal surveillance
   • Recommended: ≥2 collections on consecutive days
   • Sensitivity: >90%, Specificity: ~100% when properly collected
   • Best for: Initial screening, patients with disrupted sleep patterns

2. 24-hour Urinary Free Cortisol (UFC)

   • Advantages: Reflects integrated tissue exposure to free cortisol over 24 hours
   • Recommended: Average of 2-3 collections
   • Sensitivity: 97%, Specificity: 91% when measured by LC-MS/MS
   • Best for: Supporting diagnosis, reflecting overall cortisol production

3. Overnight 1-mg Dexamethasone Suppression Test (DST)

   • Advantages: Simple outpatient test
   • Normal response: Cortisol suppression to <50 nmol/L (1.8 μg/dL)
   • Best for: Shift workers, suspected adrenal tumors, patients with disrupted circadian rhythms
   • Caution: Avoid in women taking estrogen-containing contraceptives

Important Considerations:

• Multiple tests may be needed for diagnosis, especially in mild cases 3
• Measuring dexamethasone level along with cortisol improves DST interpretability 1, 2
• LC-MS/MS measurement of UFC provides superior accuracy compared to immunoassays 4
• For children/adolescents, unexplained weight gain combined with growth failure is a key indicator 1, 2

Determining the Source of Hypercortisolism

After confirming hypercortisolism, determine if it is ACTH-dependent or independent:

ACTH Measurement:

• Low ACTH: ACTH-independent (adrenal source)

  • Next step: Adrenal CT or MRI

• Normal or High ACTH: ACTH-dependent (pituitary or ectopic source)

  • Next step: Pituitary MRI

Further Testing Based on ACTH Status and Imaging:

For ACTH-Dependent Cases:

• Pituitary MRI findings:

  • Adenoma <6mm: Proceed to IPSS (Inferior Petrosal Sinus Sampling)
  • Adenoma 6-9mm: Consider IPSS (expert opinions differ)
  • Adenoma ≥10mm: Presumed Cushing's disease, no IPSS needed
  • No adenoma or equivocal findings: Proceed to IPSS plus whole-body CT

• CRH and DDAVP stimulation tests may be used when MRI findings are inconclusive

For ACTH-Independent Cases:

• Adrenal imaging to identify adenoma or other adrenal pathology

Pitfalls and Caveats

1. False positives can occur in:

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

2. False negatives can occur in:

   • Cyclic Cushing's syndrome (periodic hypercortisolism)
   • Mild Cushing's syndrome 3

3. Test interference:

   • Medications affecting dexamethasone metabolism (CYP3A4 inducers/inhibitors)
   • Estrogen-containing contraceptives can affect DST results
   • Inadequate saliva collection technique

Special Populations

• Children: Male predominance (63%), more aggressive disease in boys, microadenomas in 98% of cases 1
• Obese patients: Consider 2-mg overnight DST (lower false positive rate) 2
• Shift workers: DST preferred over circadian rhythm-dependent tests 2

By following this systematic approach to diagnostic testing for Cushing's disease, clinicians can efficiently confirm or exclude the diagnosis, determine the source of hypercortisolism, and guide appropriate treatment decisions to reduce morbidity and mortality associated with this condition.