Diagnostic Screening Tests for Hypercortisolism

These tests are the fundamental pillars for diagnosing Cushing's Syndrome. Because cortisol levels follow a circadian rhythm and fluctuate with stress, a single morning blood cortisol measurement is usually insufficient to confirm pathological excess.

1. 24-Hour Urinary Free Cortisol (UFC)

This test measures the total production of "free" (biologically active) cortisol excreted by the body over a full 24-hour period.

• Clinical Utility: It provides an average of cortisol production, smoothing out momentary fluctuations.

• Interpretation: A result 3 to 4 times higher than the upper limit of normal is a strong indicator of hypercortisolism.

• Limitations: It requires meticulous collection by the patient; false positives can occur due to severe anxiety, depression, or excessive alcohol consumption (Pseudo-Cushing's).

2. Late-Night Salivary Cortisol

This test takes advantage of the body's natural circadian rhythm. Under normal conditions, cortisol reaches its lowest point (nadir) around midnight.

• Clinical Utility: This is the most sensitive test for detecting the loss of the circadian rhythm, which is one of the earliest signs of Cushing's Syndrome.

• Procedure: The patient uses a collection device (salivette) between 11:00 PM and midnight.

• Advantages: It is minimally invasive and can be performed at home, avoiding the stress of a venipuncture which could artificially elevate cortisol levels.

3. Dexamethasone Suppression Test (Low Dose)

This evaluates whether the patient’s HPA axis still responds to the negative feedback mechanism.

• Procedure: The patient takes a low dose of dexamethasone (usually 1 mg) at 11:00 PM, and blood cortisol levels are measured at 8:00 AM the following morning.

• Biological Logic: In a healthy individual, dexamethasone (a potent synthetic glucocorticoid) should "trick" the pituitary gland into stopping ACTH production, resulting in very low blood cortisol levels (generally $< 1.8 µg/dL).

• Interpretation: If cortisol levels remain high, it indicates that cortisol production is autonomous and is no longer responding to the body's regulatory signals.

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Comparative Summary

Test	Primary Evaluation	Key Advantage
24h Urinary	Total daily production	Reflects continuous exposure.
Late-night Salivary	Circadian rhythm	Detects the failure to "dip" at night.
Dexamethasone	Feedback mechanism	Evaluates HPA axis autonomy.

Clinical guidelines generally recommend that at least two of these tests yield abnormal results before proceeding with localization studies (such as MRI or CT scans) to find the source of the excess (pituitary, adrenal glands, or ectopic production).