What blood work is used to diagnose Cushing's syndrome?

Blood Work for Diagnosing Cushing's Syndrome

Perform 2-3 first-line screening tests including late-night salivary cortisol, 24-hour urinary free cortisol, and overnight 1 mg dexamethasone suppression test to confirm hypercortisolism, then measure morning plasma ACTH to determine the etiology. 1, 2, 3

Initial Screening Tests for Hypercortisolism

The diagnostic approach requires multiple screening tests because no single test is perfect:

• Late-night salivary cortisol (LNSC) is the most sensitive (95%) and specific (100%) screening test, requiring at least 2-3 samples collected on consecutive days 2
• 24-hour urinary free cortisol (UFC) measures overall cortisol production with high sensitivity (89%) and specificity (100%), requiring at least 2-3 collections to account for day-to-day variability 2, 3
• Overnight 1 mg dexamethasone suppression test (DST) assesses cortisol suppressibility, with serum cortisol <1.8 μg/dL (50 nmol/L) at 8 AM after 1 mg dexamethasone at midnight indicating a normal response (sensitivity 95%, specificity 80%) 2, 3

If any screening test is abnormal, repeat 1-2 additional screening tests to confirm the diagnosis before proceeding further. 1, 2

Important Caveats for Screening Tests

• False positives occur commonly in severe obesity, uncontrolled diabetes, depression, alcoholism, and pregnancy 1, 2
• The DST may be less reliable in women taking estrogen-containing oral contraceptives 2
• Measuring dexamethasone levels alongside cortisol improves DST interpretability 1, 2
• For patients with low clinical suspicion, start with LNSC alone as it's easier for patient compliance 1

Blood Tests to Determine Etiology

Once hypercortisolism is confirmed, the next step differentiates the underlying cause:

• Morning plasma ACTH level is essential to distinguish ACTH-dependent from ACTH-independent causes 2, 3
  • Normal or elevated ACTH (>5 ng/L or >1.1 pmol/L) indicates ACTH-dependent Cushing's syndrome (pituitary adenoma or ectopic ACTH secretion) 2
  • Low or undetectable ACTH indicates ACTH-independent Cushing's syndrome (adrenal tumor or hyperplasia) 2

Additional Blood Tests for ACTH-Dependent Cases

• CRH stimulation test helps confirm pituitary origin in ACTH-dependent cases, with ≥20% increase in cortisol from baseline supporting pituitary Cushing's disease 2
• Bilateral inferior petrosal sinus sampling (BIPSS) is the gold standard when imaging is equivocal, using central-to-peripheral ACTH ratios (≥2:1 before stimulation, ≥3:1 after CRH stimulation) with 100% sensitivity 1, 2

Diagnostic Algorithm

Follow this stepwise approach:

1. Rule out exogenous glucocorticoid use as the most common cause of Cushing syndrome 1
2. Perform 2-3 first-line screening tests (LNSC, UFC, DST) based on clinical suspicion 1, 2, 3
3. If any test is abnormal, repeat 1-2 screening tests to confirm hypercortisolism 1, 2
4. Measure morning plasma ACTH once hypercortisolism is confirmed 2, 3
5. If ACTH is normal/high, proceed with pituitary MRI and consider BIPSS if imaging is negative or equivocal 1, 3
6. If ACTH is low/undetectable, proceed with adrenal imaging (CT or MRI) 2

Special Considerations

• In children, screen only if weight gain is combined with decreased height velocity or standard deviation score 2, 3
• Consider cyclic Cushing's syndrome if results are inconsistent across multiple testing occasions 2
• Scalp-hair cortisol/cortisone analysis can assess long-term glucocorticoid exposure and detect transient hypercortisolism in cyclic cases 4