Normal 24-Hour Urine Cortisol Result
The normal 24-hour urinary free cortisol excretion is typically <193 nmol/24h (<70 μg/m²/24h), with many laboratories using 40 μg (0.6 μmol) per 24 hours as the upper limit of normal. 1
Reference Values and Diagnostic Thresholds
The standard upper limit for normal 24-hour urinary free cortisol varies among clinical laboratories, but 40 μg (0.6 μmol) per 24 hours represents a better diagnostic threshold than the traditional 100 μg/24 hours cutoff 2
Values below this threshold effectively rule out Cushing's syndrome in most clinical contexts 1
The 24-hour urinary free cortisol test demonstrates >90% sensitivity for detecting Cushing's syndrome, though it has the lowest sensitivity among the three primary screening tests (late-night salivary cortisol, overnight dexamethasone suppression test, and 24-hour UFC) 1
Critical Collection Requirements
A complete 24-hour collection is essential for accurate interpretation - volume and total creatinine excretion must be measured to assess completeness of the collection 2
Spot urine specimens have too much variability and should not be utilized for cortisol assessment 2
At least 2-3 collections are recommended before making diagnostic decisions due to substantial intra-patient variability, with 24-hour urinary free cortisol able to vary by up to 50% between collections 1
Important Clinical Considerations
Renal impairment or polyuria can invalidate urinary free cortisol results, making late-night salivary cortisol a better diagnostic option in these patients 1
Copper contamination of collection apparatus must be avoided to prevent spurious results 2
Unbound cortisol is excreted unchanged in the urine, and 24-hour urinary free cortisol correlates well with mean serum-free cortisol in conditions of cortisol excess 3
Diagnostic Context for Elevated Values
Values greater than 100 μg/24 hours (1.6 μmol/24 hours) are typically diagnostic of Cushing's syndrome in symptomatic patients 2
However, overlap exists with other chronic liver diseases - patients with autoimmune hepatitis may have urinary copper excretion in the 100-200 μg/24 hours range, creating diagnostic ambiguity 2
Values just above the upper limit of normal may indicate mild Cushing's syndrome and warrant repeat testing 1