This is the gold-standard, comprehensive test for measuring your body's total cortisol production. While a blood test is a 'snapshot,' this 24-hour urine collection captures the entire day's output, averaging out all the peaks and valleys. This 'free cortisol' measurement is the most reliable and definitive way for your doctor to screen for and confirm a diagnosis of Cushing's syndrome (cortisol excess). The test requires you to collect all urine passed over a 24-hour period. This comprehensive analysis provides the most complete and accurate data, giving your doctor the definitive insight needed to manage your adrenal health.