What investigations are required to evaluate Cushing syndrome?

Investigations Required for Evaluation of Cushing Syndrome

The evaluation of Cushing syndrome requires a two-step approach: first confirming hypercortisolism, then determining its etiology through a systematic series of tests including 24-hour urinary free cortisol, late-night salivary cortisol, dexamethasone suppression testing, and ACTH measurements. 1

Step 1: Confirming Hypercortisolism

First-Line Screening Tests

• Late-night salivary cortisol (LNSC): Collect at least 2-3 samples on consecutive days; highly sensitive (95%) and specific (100%) 1
• 24-hour urinary free cortisol (UFC): Collect at least 3 samples; diagnostic cut-off >193 nmol/24h (>70 μg/m²); sensitivity 89%, specificity 100% 2, 1
• Low-dose dexamethasone suppression test (LDDST): 0.5 mg 6-hourly for 48 hours (or 30 μg/kg/day for patients <40kg); serum cortisol measured at 0,24, and 48 hours; diagnostic cut-off ≥50 nmol/L (≥1.8 μg/dL); sensitivity 95%, specificity 80% 2, 1
• Overnight dexamethasone test: 25 μg/kg at 23:00h (maximum 1 mg); serum cortisol measured at 09:00h; diagnostic cut-off ≥50 nmol/L (≥1.8 μg/dL) 2
• Serum cortisol circadian rhythm study: Measurements at 09:00h, 18:00h, and midnight (sleeping); midnight diagnostic cut-off ≥50 nmol/L (≥1.8 μg/dL); sensitivity 100%, specificity 60% 2

Important Considerations

• Rule out exogenous glucocorticoid use before biochemical testing 2, 1
• Multiple tests should be performed due to limitations of individual tests 1, 3
• False positives can occur in pseudo-Cushing states (severe obesity, uncontrolled diabetes, depression, alcoholism) 1, 3
• Consider cyclic Cushing syndrome in cases with inconsistent results 1

Step 2: Determining Etiology

ACTH-Dependent vs. ACTH-Independent

• Morning plasma ACTH level: Diagnostic cut-off >1.1 pmol/L (>5 ng/L); sensitivity 68%, specificity 100% 2, 4
  • Normal/elevated ACTH: Suggests ACTH-dependent Cushing syndrome (pituitary or ectopic source) 1, 4
  • Low/undetectable ACTH: Indicates ACTH-independent Cushing syndrome (adrenal source) 1, 4

For ACTH-Dependent Cushing Syndrome

• CRH stimulation test: 1.0 μg/kg intravenously; cortisol increase ≥20% from baseline supports pituitary origin; sensitivity 74-100% 2, 4
• Pituitary MRI scan: Adenoma detection sensitivity 63%, specificity 92% 2
• Bilateral inferior petrosal sinus sampling (BIPSS): Central-to-peripheral ACTH ratio ≥3 after CRH or desmopressin stimulation; sensitivity 100% 2, 4

For ACTH-Independent Cushing Syndrome

• Adrenal imaging (CT or MRI): To identify adrenal tumors or hyperplasia 1, 5

Diagnostic Algorithm

1. Screen with multiple first-line tests (LNSC, UFC, DST) based on clinical suspicion 1, 5
2. If screening tests are abnormal, measure plasma ACTH 1, 4
3. For normal/high ACTH (ACTH-dependent):
   • Perform pituitary MRI 1, 4
   • Consider CRH stimulation test 2, 4
   • If equivocal findings or negative imaging with high clinical suspicion, proceed with BIPSS 1, 4
4. For low ACTH (ACTH-independent):
   • Proceed with adrenal imaging 1, 5

Special Considerations

• In children and adolescents, screening should be offered only if weight gain is inexplicable and combined with either decreased height SD score or height velocity 2, 1
• Growth failure sensitively discriminates simple obesity from Cushing syndrome in prepubertal children but is unreliable in post-pubertal children 2
• Post-pubertal children should be assessed according to adult guidelines 2
• Multiple samples (urine/saliva) are needed to diagnose mild Cushing syndrome 3
• Measuring dexamethasone levels along with cortisol improves test interpretability 1