Initial Diagnostic Testing for Cushing's Syndrome
For diagnosing Cushing's syndrome, order 24-hour urinary free cortisol (UFC) as one of the initial screening tests, NOT an ACTH stimulation test. 1
Why 24-Hour UFC is the Correct Initial Test
The ACTH stimulation test is not used to diagnose Cushing's syndrome—it is used to diagnose adrenal insufficiency, which is the opposite problem. 1 The diagnostic algorithm for Cushing's syndrome begins with screening tests that detect hypercortisolism, and 24-hour UFC is one of three recommended first-line options. 1
The Three Recommended Screening Tests
According to the 2021 Endocrine Society consensus guidelines, you should choose from these initial screening tests: 1
- 24-hour urinary free cortisol (UFC): Collect 2-3 samples due to 50% random variability between collections 1
- Late-night salivary cortisol (LNSC): Obtain ≥2 samples on consecutive days 1
- Overnight 1-mg dexamethasone suppression test (DST): Single test, but less useful in shift workers or women on estrogen-containing contraceptives 1
For intermediate to high clinical suspicion, perform 2-3 different screening tests to increase diagnostic accuracy. 1
Practical Considerations for Test Selection
When to Start with 24-Hour UFC
- Suspected Cushing's disease (pituitary source): Start with either UFC and/or LNSC 1
- General screening: UFC has >90% sensitivity and provides integrated 24-hour cortisol exposure 1, 2
- Monitoring treatment response: UFC is particularly useful for tracking therapeutic efficacy 2
When UFC May Not Be Ideal
- Renal impairment (CrCl <60 mL/min): Use LNSC instead, as renal dysfunction invalidates UFC results 3
- Polyuria (>5 L/24h): LNSC is preferred 3
- Patient compliance concerns: Multiple LNSC samples may be easier for patients than complete 24-hour urine collections 1
When to Start with DST Instead
If you suspect adrenal tumor as the cause: Start with the overnight 1-mg DST rather than UFC, as LNSC has lower specificity in adrenal-source Cushing's syndrome. 1
Critical Collection Requirements for UFC
- Collect at least 2-3 samples to account for the 50% random variability between collections 1, 4
- Measure total volume and creatinine to verify collection completeness 3
- Avoid copper contamination of collection containers 3
- Diagnostic threshold: Values >100 μg/24h (1.6 μmol/24h) are typically diagnostic in symptomatic patients 3
What Happens After Positive Screening
Once you confirm hypercortisolism with screening tests, then you measure ACTH levels to determine if Cushing's syndrome is ACTH-dependent or ACTH-independent. 1, 5 This is when ACTH testing becomes relevant—not as an initial screening tool, but as a classification step after diagnosis is established.
The Diagnostic Sequence
- Screen for hypercortisolism: UFC, LNSC, or DST 1
- Confirm with repeat testing: 2-3 collections of abnormal tests 1
- Measure morning ACTH: Distinguishes ACTH-dependent from ACTH-independent causes 1, 5
- Proceed to imaging: Pituitary MRI if ACTH elevated, adrenal CT if ACTH low 1
Common Pitfalls to Avoid
- Never use ACTH stimulation testing to diagnose Cushing's syndrome—this test evaluates adrenal insufficiency, not excess 1
- Don't rely on a single UFC collection: The 50% variability means one normal result doesn't exclude mild Cushing's syndrome 6
- Exclude exogenous glucocorticoid use first: Oral, inhaled, topical, or injected steroids can confuse all test results 1
- Consider pseudo-Cushing's states: Severe obesity, uncontrolled diabetes, psychiatric disorders, and alcoholism can cause false-positive screening tests 1
- Incomplete urine collections invalidate UFC: Always verify adequate volume and creatinine excretion 3