What are the recommended tests for diagnosing Cushing's (Cushing's syndrome) syndrome?

Diagnostic Testing for Cushing's Syndrome

Start with any combination of three first-line screening tests—late-night salivary cortisol (LNSC), 24-hour urinary free cortisol (UFC), or overnight 1 mg dexamethasone suppression test (DST)—based on local availability and patient-specific factors, performing at least 2-3 measurements of each test to account for variability. 1

Initial Screening Tests

Test Selection Algorithm

For most patients with suspected Cushing's syndrome, begin with LNSC and/or UFC, as these provide the highest combined sensitivity and specificity. 1, 2

• Late-Night Salivary Cortisol (LNSC): Collect at least 2-3 samples on consecutive days at the patient's usual bedtime (typically 11 PM-midnight), with specificity of 93-100% and sensitivity of 95% 2, 3

  • Multiple LNSC collections are often easier for patient compliance than 24-hour urine collections 1, 2
  • Critical contraindication: Do NOT use in night-shift workers or anyone with disrupted sleep-wake cycles, as the test relies on normal circadian cortisol nadir 2
  • Avoid topical hydrocortisone contamination, which can falsely elevate results 2

• 24-Hour Urinary Free Cortisol (UFC): Collect 2-3 samples to account for up to 50% intra-patient variability, with sensitivity of 89-97% and specificity of 91-100% 1, 2

  • Preferred over LNSC in patients with renal impairment (CrCl <60 mL/min) or significant polyuria (>5 L/24 h) 1
  • Sex, BMI, age, urinary volume, and sodium intake can all influence results and must be considered 1

• Overnight 1 mg Dexamethasone Suppression Test (DST): Administer 1 mg dexamethasone at 11 PM-midnight, measure serum cortisol at 8 AM; normal response is <1.8 μg/dL (50 nmol/L), with sensitivity of 95-100% and specificity of 90-97% 2, 4, 5

  • Preferred test for shift workers and patients with disrupted circadian rhythms 1
  • Not reliable in women taking oral estrogen-containing contraceptives due to increased corticosteroid-binding globulin 1
  • Measuring dexamethasone levels concomitantly with cortisol improves test interpretability and reduces false-positives 1, 3

Special Scenario: Suspected Adrenal Tumor

If an adrenal tumor is suspected, start with DST and only use LNSC if cortisone levels can also be reported, as LNSC has lower specificity in adrenal Cushing's syndrome. 1

Confirming the Diagnosis

If any screening test is abnormal, repeat 1-2 screening tests to confirm hypercortisolism before proceeding with further workup. 1, 2, 3

• If all tests are normal and clinical suspicion is low, Cushing's syndrome is unlikely 2, 3
• Common pitfall: Pseudo-Cushing's states (severe obesity, uncontrolled diabetes, depression, alcoholism, pregnancy, PCOS) can cause mildly elevated results, typically with UFC <3-fold above normal 1, 2

Distinguishing Pseudo-Cushing's from True Cushing's Syndrome

When pseudo-Cushing's is suspected:

• Consider the patient's clinical history, particularly duration of symptoms, and repeat testing over 3-6 months 1
• Use the desmopressin test (easier, less expensive) or Dex-CRH test (more complex but validated) to distinguish ACTH-dependent Cushing's from pseudo-Cushing's 1
• Both tests show good diagnostic performance and excellent agreement when performed together 1

Determining Etiology

Once hypercortisolism is confirmed, measure morning plasma ACTH to differentiate ACTH-dependent from ACTH-independent causes. 2, 3, 4

ACTH Results Interpretation

• Low/undetectable ACTH (<5 ng/L or <1.1 pmol/L): ACTH-independent (adrenal) Cushing's syndrome → proceed with adrenal CT or MRI 2, 3
• Normal/elevated ACTH (≥5 ng/L or ≥1.1 pmol/L): ACTH-dependent Cushing's syndrome → proceed with pituitary MRI 2, 3, 4

Pituitary MRI Findings in ACTH-Dependent Disease

• Lesion ≥10 mm: Cushing's disease is presumed; proceed directly to transsphenoidal surgery 1, 3, 4
• Lesion 6-9 mm: Expert opinions differ; consider CRH/desmopressin testing or bilateral inferior petrosal sinus sampling (BIPSS) 1
• Lesion <6 mm or negative MRI: Perform BIPSS to differentiate pituitary from ectopic ACTH secretion 1, 3, 4

Bilateral Inferior Petrosal Sinus Sampling (BIPSS)

BIPSS should NOT be used to diagnose hypercortisolism itself, only to localize the source of ACTH in confirmed ACTH-dependent Cushing's syndrome. 1

• Diagnostic criteria: central-to-peripheral ACTH ratio ≥2:1 before CRH stimulation and ≥3:1 after CRH stimulation, with 100% sensitivity 3, 4
• In cyclic Cushing's disease, confirm active hypercortisolism with LNSC, DST, or UFC immediately before BIPSS 1

Critical Pitfalls to Avoid

• Never perform LNSC in shift workers or those with irregular sleep schedules—the test becomes unreliable 2
• Do not use IPSS to diagnose hypercortisolism—it only localizes ACTH source after diagnosis is confirmed 1
• Measure dexamethasone levels with DST when false-positives are suspected (e.g., medications affecting dexamethasone metabolism) 1, 3
• Account for medications: CYP3A4 inducers (phenobarbital, carbamazepine) can cause false-negative DST; fluoxetine, cimetidine, diltiazem can cause false-positive DST 1
• In cyclic Cushing's syndrome, multiple sequential measurements over time are essential, as patients may have weeks to months of normal cortisol between episodes 2