What tests are used to diagnose Cushing disease?

Diagnostic Testing for Cushing Disease

Start with any combination of three first-line screening tests: overnight 1 mg dexamethasone suppression test (DST), 24-hour urinary free cortisol (UFC) measured 2-3 times, or late-night salivary cortisol (LNSC) measured 2-3 times—there is no single preferred test, and selection depends on patient-specific factors and local availability 1.

Initial Screening Tests

The 2021 Lancet consensus guideline establishes that clinical judgment drives test selection based on individual circumstances 1:

• DST (1 mg overnight): Preferred for shift workers and those with disrupted sleep schedules. Post-dexamethasone cortisol >5 μg/dL indicates dysregulated cortisol secretion. Measure dexamethasone levels simultaneously if false-positive suspected to account for malabsorption or drug interactions with CYP3A4 inducers 1.

• UFC (24-hour collections): Obtain 2-3 collections to account for up to 50% intra-patient variability. Avoid in patients with renal impairment (CrCl <60 mL/min) or polyuria (>5 L/24h) as these strongly affect results 1. Independent of corticosteroid-binding globulin (CBG) changes, making it useful when oral estrogens confound DST.

• LNSC: Collect 2-3 samples. Easier for patients to complete than UFC. If adrenal tumor suspected, only use LNSC if cortisone levels can also be measured 1.

Critical Pitfall: Pseudo-Cushing Syndrome

Psychiatric disorders, alcohol use, PCOS, and obesity can activate the HPA axis, producing mildly elevated results (UFC typically <3-fold normal). If screening tests are equivocal, use the desmopressin test or combined dexamethasone-CRH test to distinguish true Cushing syndrome from pseudo-Cushing 1. The desmopressin test is less complex and expensive while maintaining high specificity for Cushing disease.

Establishing ACTH-Dependent Disease

Once hypercortisolism is confirmed, measure plasma ACTH to differentiate ACTH-dependent (normal/high ACTH) from ACTH-independent (low ACTH) causes 1.

For ACTH-dependent disease:

• Obtain pituitary MRI to identify adenomas
• CRH stimulation test: 100% sensitive for microadenomas, 73% for macroadenomas in Cushing disease; only 27% positive in ectopic ACTH syndrome 2
• Desmopressin test: ACTH increases in 86% of Cushing disease cases (90% in microadenomas) versus 44% in ectopic ACTH syndrome 2

Distinguishing Cushing Disease from Ectopic ACTH

The high-dose dexamethasone suppression test (8 mg) suppresses morning cortisol in 89% of microadenomas and 82% of all Cushing disease cases, but only 20% of ectopic ACTH syndrome 2. When combined with CRH testing, these achieve 81% sensitivity for Cushing disease 2.

Role of Inferior Petrosal Sinus Sampling (IPSS)

IPSS should NOT be used to diagnose hypercortisolism itself—only to localize the source after ACTH-dependent disease is confirmed 1. The 2021 guideline provides clear thresholds:

• All patients with lesions <6 mm on MRI should undergo IPSS
• Patients with lesions ≥10 mm do not need IPSS
• For lesions 6-9 mm, expert opinion differs—consider IPSS based on biochemical test results 1

IPSS correctly identifies pituitary source in 88% of cases 3 and has high specificity when performed at experienced centers 4.

Practical Algorithm Summary

1. Screen with 2-3 tests (DST, UFC, or LNSC based on patient factors)
2. If positive and clinical suspicion high: Measure ACTH
3. If ACTH normal/high: Obtain pituitary MRI
4. Perform CRH and/or desmopressin testing to confirm pituitary source
5. IPSS if MRI shows <6 mm lesion or equivocal findings
6. If IPSS or imaging inconclusive: Whole-body CT to exclude ectopic source 1

Special Consideration: Cyclic Cushing Disease

In patients with fluctuating cortisol levels, confirm active hypercortisolism with LNSC, DST, or UFC immediately before dynamic testing or IPSS to ensure testing during the active phase 1.

The non-invasive strategy combining CRH test (cortisol increase >17%, ACTH increase >37%), desmopressin test (cortisol increase >18%, ACTH increase >33%), pituitary MRI, and whole-body CT can achieve 100% positive predictive value and potentially avoid IPSS in 47% of cases where it's currently recommended 4.