What is the procedure for collecting a 24-hour urine cortisol test in a patient suspected of having Cushing's syndrome?

24-Hour Urine Cortisol Collection Procedure

The 24-hour urine cortisol test requires collecting all urine produced over a complete 24-hour period in a special container, with the collection typically starting in the morning by discarding the first void and then collecting every subsequent void until the same time the next morning, including that final void. 1

Collection Protocol

Timing and Duration

• Begin the collection in the morning (typically at 0800h) by emptying the bladder completely and discarding this first void 1
• Collect every single urine void for the next 24 hours in the provided container 1
• End the collection exactly 24 hours later by including the final morning void at the same time (0800h the next day) 1

Critical Collection Requirements

Complete collection is absolutely essential for accurate interpretation, and the following must be measured to verify adequacy: 2

• Total urine volume over the 24-hour period 2
• Total creatinine excretion to assess completeness of collection 2
• Inadequate creatinine excretion indicates an incomplete collection and invalidates the results 2

Important Precautions

• Avoid copper contamination of the collection container, as this can produce spurious results 2
• Keep the collection container refrigerated or on ice throughout the 24-hour period (standard practice, though not explicitly stated in guidelines)
• Ensure the patient understands that missing even a single void invalidates the entire collection 2

Diagnostic Thresholds

Pediatric Populations

• Values >193 nmol/24h (>70 μg/m²) are diagnostic with 89% sensitivity and 100% specificity 1
• This test should be performed on 3 consecutive days for optimal diagnostic accuracy 1

Adult Populations

• The diagnostic threshold of >40 μg (0.6 μmol) per 24 hours represents a better cutoff than the traditional 100 μg/24h 2
• Values >100 μg/24h (1.6 μmol/24h) are typically diagnostic of Cushing's syndrome in symptomatic patients 2

Test Characteristics and Limitations

Variability Considerations

• 24-hour UFC can vary by up to 50% between collections, which is why at least 2-3 collections are recommended before making diagnostic decisions 2, 3
• This high random variability means a single normal result does not exclude Cushing's syndrome, particularly in cyclic disease 4

When This Test May Be Invalid

• Renal impairment or polyuria can invalidate UFC results, making late-night salivary cortisol a better alternative in these patients 2
• Incomplete collections (verified by low creatinine excretion) cannot be interpreted 2

Clinical Context

Role in Diagnostic Algorithm

The 24-hour UFC is one of three recommended first-line screening tests for Cushing's syndrome, alongside late-night salivary cortisol and dexamethasone suppression testing 1, 5. Among these three tests, UFC has sensitivity >90% but the lowest specificity, making it important to combine with other screening modalities 2.

Special Populations

In children and adolescents with suspected Cushing's disease, the 24-hour UFC demonstrates 89% sensitivity and 100% specificity when the appropriate pediatric threshold is used 1. However, the test must be repeated on 3 separate days to account for day-to-day variability and the possibility of cyclic hypercortisolism 1.