Diagnostic Approach to Cushing's Syndrome

Initial Screening Tests

The Endocrine Society recommends using at least two of three first-line screening tests, including 24-hour urinary free cortisol (UFC), late-night salivary cortisol (LNSC), or overnight 1-mg dexamethasone suppression test (DST), to confirm Cushing's syndrome, with UFC and LNSC being preferred for their high sensitivity (>90%) and specificity (approaching 100%) 1
24-hour urinary free cortisol (UFC) collection for 3 consecutive days with diagnostic cut-off >193 nmol/24h (>70 μg/m²) demonstrates 89% sensitivity and 100% specificity 1
Overnight 1-mg dexamethasone suppression test with serum cortisol >1.8 μg/dL (50 nmol/L) at 8 AM after 1 mg dexamethasone given at midnight indicates Cushing's syndrome 1

Perform at least 2-3 abnormal screening tests before confirming the diagnosis, as no single test achieves 100% diagnostic accuracy 1

UFC excretion for 3 days: >193 nmol/24h (>70 μg/m²) with 89% sensitivity and 100% specificity 1
Serum cortisol circadian rhythm: Midnight sleeping cortisol ≥50 nmol/L (≥1.8 μg/dL) with 100% sensitivity and 60% specificity 1
Late-night salivary cortisol: Based on local assay cut-off with 95% sensitivity and 100% specificity 1
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 1

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) 1

The Endocrine Society recommends using 24-hour urinary free cortisol (UFC) as one of the initial screening tests for diagnosing Cushing's syndrome, rather than an ACTH stimulation test, due to its high sensitivity and ability to provide integrated 24-hour cortisol exposure 2, 3

The Endocrine Society consensus guidelines suggest choosing from three initial screening tests: 24-hour urinary free cortisol (UFC), late-night salivary cortisol (LNSC), and overnight 1-mg dexamethasone suppression test (DST), with UFC having >90% sensitivity 2, 3
For suspected Cushing's disease (pituitary source), start with either UFC and/or LNSC, as they are more sensitive for detecting pituitary-related Cushing's syndrome 2

When to start with 24-hour UFC: general screening, as it provides integrated 24-hour cortisol exposure, and has >90% sensitivity 2, 3
When UFC may not be ideal: renal impairment (CrCl <60 mL/min), polyuria (>5 L/24h), or patient compliance concerns, in which case LNSC is preferred 4
Diagnostic threshold for UFC: values >100 μg/24h (1.6 μmol/24h) are typically diagnostic in symptomatic patients 4

The diagnostic sequence for Cushing's syndrome involves screening for hypercortisolism using UFC, LNSC, or DST, followed by confirmation with repeat testing, measurement of morning ACTH, and imaging studies 2, 3, 5
Common pitfalls to avoid include using ACTH stimulation testing to diagnose Cushing's syndrome, relying on a single UFC collection, and not excluding exogenous glucocorticoid use before testing 2, 3