Diagnostic Workup for Cushing Syndrome
Initial Step: Exclude Exogenous Glucocorticoid Exposure
Before any biochemical testing, eliminate all exogenous glucocorticoid sources, as exogenous steroids are the most common cause of Cushing syndrome. 1, 2 This includes oral, injectable, inhaled, and topical glucocorticoid preparations. 3
Step 1: Confirm Hypercortisolism with First-Line Screening Tests
Perform 2-3 first-line screening tests simultaneously rather than relying on a single test, as no test has 100% diagnostic accuracy. 1, 4 The three essential tests are:
24-hour urinary free cortisol (UFC) collected over 3 consecutive days, with diagnostic cut-off >193 nmol/24 h (>70 μg/m²), sensitivity 89-95%, specificity 98-100% 5, 1, 6
Late-night salivary cortisol (most useful screening test), with sensitivity 95% and specificity 100% based on local assay cut-offs 5, 1, 6
Overnight 1 mg dexamethasone suppression test (or 25 μg/kg in children <40 kg, maximum 1 mg), with serum cortisol measured at 09:00 h; failure to suppress to <50 nmol/L (<1.8 μg/dL) indicates Cushing syndrome, with sensitivity 95% and specificity 80-90% 5, 1, 4
Alternatively, a low-dose dexamethasone suppression test (0.5 mg every 6 hours for 48 hours) can be performed, measuring serum cortisol at 0,24, and 48 hours, with failure to suppress to <50 nmol/L indicating hypercortisolism. 5
Late-night salivary cortisol appears to be the most useful screening test, particularly in children as it is non-invasive. 1, 6 UFC and dexamethasone suppression testing should provide further confirmation. 6
Step 2: Determine ACTH-Dependency
Once hypercortisolism is confirmed, measure morning (09:00 h) plasma ACTH level to differentiate ACTH-dependent from ACTH-independent causes. 5, 2, 4
ACTH >1.1 pmol/L (>5 ng/L) indicates ACTH-dependent Cushing syndrome (pituitary adenoma or ectopic ACTH secretion), with sensitivity 68-80% and specificity 90-100% 5, 1, 7
Suppressed or low ACTH indicates ACTH-independent Cushing syndrome (adrenal adenoma, carcinoma, or bilateral hyperplasia) 4, 7, 8
Step 3: Localize the Source of ACTH-Dependent Disease
Pituitary MRI Imaging
Obtain pituitary MRI with gadolinium contrast to identify corticotroph adenomas. 5, 2, 4 MRI has sensitivity 63% and specificity 92% for detecting pituitary adenomas. 5, 1
If a lesion ≥10 mm is identified on MRI, it is presumed to be Cushing disease with 90% positive predictive value. 1
Microadenomas (≤2 mm) account for 98% of pediatric Cushing disease cases, making detection challenging. 5, 1
Bilateral Inferior Petrosal Sinus Sampling (BIPSS)
Perform BIPSS with CRH or desmopressin stimulation in almost all patients with ACTH-dependent Cushing syndrome, except those with a pituitary macroadenoma ≥10 mm. 5, 2, 7 This is the gold standard for differentiating pituitary from ectopic ACTH secretion. 2, 4
Central-to-peripheral ACTH ratio ≥3 after CRH or desmopressin stimulation indicates pituitary Cushing disease with 100% sensitivity 5, 1
Additional Dynamic Tests (if BIPSS unavailable or contraindicated)
CRH stimulation test: Cortisol increase ≥20% suggests pituitary source, with sensitivity 74-100% 5
High-dose dexamethasone suppression test: >50% suppression of cortisol or UFC suggests pituitary disease, though this test has lower accuracy than BIPSS 4, 7
Step 4: Imaging for Ectopic ACTH or Adrenal Sources
For Suspected Ectopic ACTH Secretion
Obtain chest CT and abdominal CT to identify ectopic ACTH-secreting tumors (bronchial carcinoid, small cell lung cancer, pancreatic neuroendocrine tumors, thymic carcinoids). 5, 2, 4
For ACTH-Independent Disease
Perform adrenal CT or MRI to characterize adrenal masses and differentiate adenoma from carcinoma or bilateral hyperplasia. 4, 7, 8
Pediatric-Specific Considerations
In children with obesity, screen for Cushing syndrome only if weight gain is unexplained AND combined with either declining height velocity or decreasing height standard deviation score (SDS). 5, 1 This combination has high sensitivity and specificity for Cushing syndrome in children. 5, 1
Growth failure (height SDS below mean while BMI SDS rises above mean) sensitively discriminates Cushing syndrome from simple obesity in prepubertal children. 5, 1
Wide purple striae are a distinguishing feature, though absence does not exclude diagnosis. 1, 6
Boys with Cushing disease tend to have more aggressive disease with higher BMI, shorter height, and higher ACTH levels than girls. 5, 1
Common Pitfalls to Avoid
Do not rely on a single screening test—perform 2-3 tests simultaneously as each has limitations. 1, 4
Do not proceed with etiologic workup until hypercortisolism is definitively confirmed, as false positives lead to unnecessary invasive testing. 2, 4
Do not assume all obese children require Cushing syndrome screening—only those with unexplained weight gain plus growth deceleration warrant investigation. 5, 1
Do not skip BIPSS in ACTH-dependent cases without a clear pituitary macroadenoma, as it is the most accurate test for distinguishing pituitary from ectopic sources. 2, 7
Be aware of cyclical or periodic hypercortisolism, which can produce false-negative results; repeat testing may be necessary if clinical suspicion remains high. 6, 4