What is the stepwise diagnostic approach for confirming Cushing disease (ACTH‑secreting pituitary adenoma) in an adult presenting with signs of hypercortisolism such as central obesity, moon face, dorsocervical fat pad, skin thinning, easy bruising, violaceous striae, hypertension, glucose intolerance, or osteoporosis?

Diagnosis of Cushing Disease: Stepwise Algorithmic Approach

In an adult with clinical features of hypercortisolism, confirm biochemical hypercortisolism first using 2–3 measurements of at least two screening tests (24-hour urinary free cortisol, late-night salivary cortisol, or 1-mg overnight dexamethasone suppression test), then measure morning plasma ACTH to establish ACTH-dependency, followed by pituitary MRI, and proceed to bilateral inferior petrosal sinus sampling (BIPSS) if the MRI shows no adenoma or a lesion <6 mm. 1

Step 1: Confirm Biochemical Hypercortisolism

Before pursuing any localization studies, you must document autonomous cortisol excess using at least 2–3 measurements of a minimum of two different screening tests to account for biological variability and cyclic disease patterns. 1, 2

First-Line Screening Tests (Choose ≥2):

• 24-hour urinary free cortisol (UFC): Values >100 μg/24 hours on multiple collections are diagnostic; however, values >40 μg/24 hours warrant further evaluation. Ensure complete collection by measuring total volume and creatinine excretion. 1, 2

• Late-night salivary cortisol (LNSC): Collect between 2300–2400 hours on 2–3 separate nights. Abnormal threshold is >3.6 nmol/L (>0.1 μg/dL), with sensitivity 92–100% and specificity 93–100%. This test captures loss of normal circadian rhythm, a hallmark of Cushing's syndrome. 1, 2

• 1-mg overnight dexamethasone suppression test (DST): Administer 1 mg dexamethasone at 2300–2400 hours, measure serum cortisol at 0800 hours the next morning. Abnormal result is cortisol ≥1.8 μg/dL (≥50 nmol/L); values >5 μg/dL indicate overt disease. Consider measuring dexamethasone levels concomitantly to exclude false-positive results from abnormal drug metabolism. 1, 2

Critical Pitfalls to Avoid:

• Exclude exogenous glucocorticoid exposure (oral, inhaled fluticasone, topical, intra-articular/epidural injections) before any testing—failure to do so leads to unnecessary investigations. 2

• Recognize pseudo-Cushing's states: Severe obesity, depression, alcoholism, polycystic ovary syndrome, and uncontrolled diabetes can cause mild hypercortisolism that mimics true Cushing's syndrome. 1, 2

• Oral contraceptives/estrogen therapy increase cortisol-binding globulin, falsely elevating total cortisol without true hypercortisolism. 2

• CYP3A4 inducers (phenytoin, rifampin, carbamazepine) accelerate dexamethasone metabolism, causing false-positive DST results. 1, 2

If Screening Results Are Equivocal:

• Perform the Dex-CRH test (dexamethasone suppression followed by CRH stimulation): A cortisol rise >38 nmol/L at 15 minutes post-CRH indicates true Cushing's disease with 90% sensitivity and 95% specificity. 1

• Consider serial monitoring over 3–6 months if cyclic Cushing's syndrome is suspected, as patients may have weeks to months of normal cortisol interspersed with hypercortisolism. 1, 2

Step 2: Determine ACTH-Dependency

Once hypercortisolism is confirmed, measure morning (0800–0900 hours) plasma ACTH to distinguish ACTH-dependent from ACTH-independent causes. 1, 3

ACTH Interpretation:

• ACTH >5 ng/L (>1.1 pmol/L): Indicates ACTH-dependent Cushing's syndrome (pituitary adenoma or ectopic ACTH source). Any detectable ACTH in the setting of confirmed hypercortisolism suggests ACTH-dependency with high certainty. 1, 3

• ACTH >29 ng/L (>6.4 pmol/L): Has 70% sensitivity and 100% specificity for diagnosing Cushing's disease specifically (pituitary source). 4, 1, 3

• ACTH <5 ng/L or undetectable: Indicates ACTH-independent Cushing's syndrome (adrenal adenoma, carcinoma, or hyperplasia). Proceed directly to adrenal CT or MRI. 1, 3

Key Point:

Morning timing is essential because ACTH follows a circadian rhythm with peak levels in the morning; afternoon measurements are unreliable and do not correspond to established diagnostic thresholds. 1

Step 3: Pituitary MRI for ACTH-Dependent Disease

For confirmed ACTH-dependent Cushing's syndrome, obtain high-quality pituitary MRI with thin slices (3T MRI preferred over 1.5T) to identify a pituitary adenoma. 1

MRI Interpretation and Next Steps:

• Adenoma ≥10 mm: Strongly suggests Cushing's disease; proceed directly to transsphenoidal surgery without further testing. 1

• Adenoma 6–9 mm: Consider CRH stimulation test or proceed to BIPSS for confirmation before surgery. 1

• No adenoma or lesion <6 mm: MRI has only 63% sensitivity for detecting ACTH-secreting pituitary adenomas (microadenomas are frequently ≤2 mm). BIPSS is mandatory to distinguish pituitary from ectopic ACTH sources. 1, 5

Step 4: Bilateral Inferior Petrosal Sinus Sampling (BIPSS)

BIPSS is the gold standard for differentiating pituitary Cushing's disease from ectopic ACTH syndrome when MRI is inconclusive or shows no adenoma. 4, 1, 3

When to Perform BIPSS:

• ACTH-dependent Cushing's syndrome with no identified adenoma on pituitary MRI 4
• Pituitary lesion <6 mm on MRI 1
• Equivocal CRH stimulation test results 1

BIPSS Diagnostic Criteria:

• Central-to-peripheral ACTH ratio ≥2:1 at baseline or ≥3:1 after CRH or desmopressin stimulation confirms a pituitary source, with sensitivity 96–100% and specificity approaching 100%. 4, 1, 3

• Inter-petrosal sinus ACTH gradient ≥1.4 after stimulation may indicate tumor lateralization, though this has only 58–87.5% concordance with surgical findings. 4

Critical Requirements for BIPSS:

• Confirm active hypercortisolism immediately before BIPSS (same morning) to ensure patients with cyclic disease are in an active phase. 4, 1

• Stop all steroidogenesis inhibitors before BIPSS; duration depends on drug half-life. 4

• Perform only at specialized centers with experienced interventional radiologists who regularly perform this procedure in adults, as reliability and complication rates are operator-dependent. 4, 1

• Measure prolactin levels simultaneously from petrosal sinuses to confirm adequate venous efflux and improve diagnostic accuracy. 1

If BIPSS Confirms Pituitary Source:

Proceed to transsphenoidal adenoma resection. 5

If BIPSS Suggests Ectopic ACTH:

• Perform neck-to-pelvis thin-slice CT scan to localize ectopic ACTH-secreting tumor (bronchial carcinoid, thymic carcinoid, pancreatic neuroendocrine tumor). 1

• Consider 68Ga-DOTATATE PET imaging for neuroendocrine tumors not visible on conventional imaging. 1

Additional Diagnostic Considerations

CRH or Desmopressin Stimulation Test:

Can be used as a non-invasive alternative when BIPSS is unavailable or contraindicated. Increased plasma ACTH and cortisol following CRH administration usually indicates Cushing's disease, while ectopic ACTH-secreting tumors show blunted responses. However, this test does not reach 100% specificity, and results may be discordant in up to one-third of patients. 1

High-Dose Dexamethasone Suppression Test:

Historically used but less reliable than BIPSS; not recommended as a primary diagnostic tool for localization. 6, 7, 8

Summary Algorithm

1. Screen with ≥2 tests (UFC, LNSC, DST), obtain 2–3 measurements each → Confirm hypercortisolism 1, 2
2. Measure morning plasma ACTH → Determine ACTH-dependency 1, 3
3. If ACTH >5 ng/L → Pituitary MRI 1
4. If MRI shows adenoma ≥10 mm → Surgery 1
5. If MRI shows no adenoma or <6 mm lesion → BIPSS 4, 1
6. If BIPSS confirms pituitary source → Surgery 5
7. If BIPSS suggests ectopic source → CT/PET imaging 1

None of the diagnostic tests reach 100% specificity, and results may be discordant in up to one-third of patients; clinical context and test results must be integrated to guide management. 1