II. Indications: Cushing's Syndrome Screening
- Preferred among 3 first line options for Cushing's Syndrome Screening
III. Mechanism
- Measures Urine 17-Ketosteroid Excretion and Urine 17-Hydroxysteroid Excretion
IV. Technique
- Collect urine for 24 hours
- Start in the morning after first void
- Continue to collect urine until the next morning and include the first morning void
- Obtain urine Cortisol and Urine Creatinine
- Perform at least twice due to variability (discrepant in 50% of samples)
V. Interpretation: Normal
- Free Urine Cortisol: 10-84 mcg/24 hours (up to 110 mcg/24 hours in some reference ranges)
VI. Causes: Increased (especially if 4x normal range)
VII. Causes: False Positives
- Incorrect urine collection
- Pregnancy
- Excessive fluid intake (>5 Liters/day)
- Pseudo-Cushing's Syndrome
-
Drug Interactions
- Carbamazepine
- Fenofibrate
- Estrogen
- Licorice
- Carbenoxolone
VIII. Causes: False Negatives
- Incorrect urine collection
- Impaired Renal Function (eGFR <60 ml/min)
- Low level Cushing's Syndrome
IX. Efficacy
- Relies on complete and accurate urine collection
- Often falsely normal in adrenal hypercortisolism (use Dexamethasone suppression instead)
- Perform at least twice due to variability (discrepant in 50% of samples)
- Test Sensitivity: 80 to 98%
- Test Specificity: 45 to 98%