Confirmatory Testing for Cushing's Syndrome
First-Line Screening Tests
When Cushing's syndrome is suspected, begin with 2-3 first-line screening tests: 24-hour urinary free cortisol (UFC), overnight 1 mg dexamethasone suppression test (DST), and/or late-night salivary cortisol (LNSC), with at least two abnormal results required to confirm hypercortisolism. 1
Test Selection Based on Clinical Context
For most patients with intermediate to high clinical suspicion, perform 2-3 of the following tests to maximize diagnostic accuracy 1, 2:
For shift workers or patients with disrupted circadian rhythm, DST may be preferred over LNSC since circadian rhythm testing is unreliable in these populations 1
For women on oral estrogen, DST may yield false positives due to increased corticosteroid-binding globulin; LNSC or UFC are preferred 1
For suspected adrenal tumors, start with DST and only use LNSC if cortisone levels can also be measured 1
Test Repetition Requirements
If UFC is used, obtain 2-3 collections to evaluate variability, as single measurements may miss intermittent hypercortisolism 1
If LNSC is used, perform at least 2-3 tests to account for day-to-day variability 1
Measure dexamethasone levels simultaneously with DST if false-positive results are suspected due to malabsorption or drug interactions with CYP3A4 inducers 1, 2
Ruling Out Pseudo-Cushing's States
Patients with mild hypercortisolism (UFC typically <3-fold normal) and conditions like obesity, depression, or alcoholism require additional testing to distinguish true Cushing's syndrome from pseudo-Cushing's states. 1
Distinguishing Tests
Dexamethasone-CRH (Dex-CRH) test: Cortisol rise >38 nmol/L at 15 minutes after CRH administration indicates true Cushing's syndrome with 90% sensitivity and 95% specificity 5, 2
Desmopressin stimulation test: Shows high specificity for Cushing's disease and demonstrates excellent agreement with Dex-CRH testing 1
Low-dose dexamethasone suppression test (LDDT) or serial LNSC over time can help differentiate, particularly when correlated with clinical picture 1
Clinical Monitoring Approach
For equivocal cases with mild hypercortisolism, monitor for 3-6 months to assess whether symptoms resolve spontaneously 1
Treat underlying conditions (such as depression or alcoholism) as these can restore normal HPA axis function and cortisol levels 1
Common Pitfalls and Caveats
False Positive Causes
Severe obesity, uncontrolled diabetes, depression, and alcoholism can all cause false positive screening results 1, 2
Medications affecting dexamethasone metabolism (CYP3A4 inducers like phenytoin, rifampin) can cause false-positive DST 5, 3
Oral estrogen or pregnancy increases corticosteroid-binding globulin, leading to false-positive DST results 1
Special Considerations
Cyclic Cushing's syndrome produces inconsistent results; repeat testing during symptomatic periods and document active hypercortisolism with LNSC, DST, or UFC before proceeding to localization studies 1
Bilateral inferior petrosal sinus sampling (IPSS) should NOT be used to diagnose hypercortisolism as it is a localization test, not a confirmatory test for cortisol excess 1
No single test reaches 100% specificity, and results may be discordant in up to one-third of patients, requiring clinical judgment and repeat testing 5
Diagnostic Algorithm Summary
- Exclude exogenous glucocorticoid use as the most common cause 2
- Perform 2-3 first-line screening tests (UFC, DST, LNSC) based on patient characteristics 1, 2
- If any test is abnormal, repeat 1-2 screening tests to confirm hypercortisolism 2
- If results remain equivocal or suggest pseudo-Cushing's, perform Dex-CRH or desmopressin testing 1, 5
- Once hypercortisolism is confirmed, measure morning plasma ACTH to determine ACTH-dependent vs. ACTH-independent etiology 5, 2, 3