What are the diagnostic tests for Cushing's disease?

Diagnostic Testing for Cushing's Disease

Start with any combination of three first-line screening tests—late-night salivary cortisol (LNSC), 24-hour urinary free cortisol (UFC), or overnight dexamethasone suppression test (DST)—and perform at least 2-3 measurements of each test you choose to confirm hypercortisolism before proceeding to determine the underlying cause. 1

Initial Screening Tests for Hypercortisolism

The diagnosis requires demonstrating pathologic cortisol excess through biochemical testing. No single test is perfect, so multiple tests are recommended. 1

Late-Night Salivary Cortisol (LNSC)

• Collect at least 2-3 samples on consecutive days to account for variability 1
• Sensitivity 95%, specificity 100% 2
• May be the easiest option for patient compliance compared to 24-hour urine collections 2
• Measures loss of normal circadian rhythm (cortisol should be lowest at night) 3
• Less useful if adrenal tumor suspected unless cortisone levels can also be measured 1

24-Hour Urinary Free Cortisol (UFC)

• Collect 2-3 separate 24-hour urine samples to evaluate day-to-day variability 1
• Sensitivity 89%, specificity 100% 2
• Diagnostic cut-off: >193 nmol/24h (>70 μg/m²) in children 1, 4
• Avoid in patients with renal impairment (CrCl <60 mL/min) or significant polyuria (>5 L/24h) as these conditions affect results 1

Overnight Dexamethasone Suppression Test (DST)

• Give 1 mg dexamethasone at midnight, measure serum cortisol at 8 AM 1
• Normal response: cortisol <1.8 μg/dL (50 nmol/L) 2
• Sensitivity 95%, specificity 80% 4
• Preferred for shift workers and patients with disrupted sleep schedules 1
• Unreliable in women taking estrogen-containing oral contraceptives 1, 2
• Consider measuring dexamethasone level alongside cortisol to rule out false-positives from poor absorption or rapid metabolism 1, 2

Determining the Cause: ACTH-Dependent vs ACTH-Independent

Once hypercortisolism is confirmed, measure morning (9 AM) plasma ACTH to differentiate the source: 1, 2, 4

ACTH-Dependent (Normal or Elevated ACTH >5 ng/L or >1.1 pmol/L)

• Indicates pituitary Cushing's disease (70-80% of cases) or ectopic ACTH secretion (15%) 1, 5
• Proceed with pituitary MRI to look for adenoma 1, 4
• If pituitary MRI shows adenoma ≥10 mm: presumed Cushing's disease, no further localization needed 1
• If adenoma <6 mm or MRI negative: perform bilateral inferior petrosal sinus sampling (IPSS) to distinguish pituitary from ectopic ACTH sources 1, 4
• For adenomas 6-9 mm: expert opinions differ, but IPSS is generally recommended 1
• IPSS should NOT be used to diagnose hypercortisolism itself—only to localize the source after hypercortisolism is confirmed 1

ACTH-Independent (Low or Undetectable ACTH)

• Indicates adrenal tumor (15-20% of cases) 4, 5
• Proceed with adrenal CT or MRI 1, 6

Special Considerations and Pitfalls

Pseudo-Cushing's States

Conditions that can cause false-positive results include: 2, 6

• Severe obesity
• Uncontrolled diabetes mellitus
• Depression and psychiatric disorders
• Alcoholism
• Polycystic ovary syndrome

To distinguish pseudo-Cushing's from true Cushing's syndrome: 1

• Consider clinical history and duration of symptoms
• Repeat testing after 3-6 months or after treating underlying condition
• Use dexamethasone-CRH test or desmopressin test (both show good diagnostic performance)
• Pseudo-Cushing's typically shows only mild elevations (UFC almost always <3-fold normal) 1

Cyclic Cushing's Syndrome

• If results are inconsistent, suspect cyclic disease 2, 6
• Perform dynamic testing and IPSS only when confirmatory tests document active hypercortisolism phase 1

Pediatric Considerations

• Screen children ONLY if unexplained weight gain is combined with either decreased height velocity or declining height percentile 1, 6
• This combination has high sensitivity and specificity for Cushing's syndrome in children 1
• Growth failure reliably distinguishes Cushing's from simple obesity in prepubertal children but is unreliable in post-pubertal adolescents 1, 6
• Cushing's disease accounts for 75-80% of pediatric cases (vs. adrenal causes more common in younger children) 1, 6

Critical Testing Principles

• Never rely on a single abnormal test result—always confirm with repeat testing 6
• Eliminate exogenous glucocorticoid use before any biochemical testing 1, 6
• If all screening tests are normal, Cushing's syndrome is unlikely 1, 2
• No preference exists between mass spectrometry and immunoassay for cortisol measurement 1