24-Hour Urine Collection Method
Collection Procedure
The proper 24-hour urine collection begins by having the patient empty their bladder and discard that urine at the start time, then collect all subsequent urine for exactly 24 hours, ending by emptying the bladder just before the 24-hour period ends and including that final void in the collection. 1
Step-by-Step Instructions
Start the collection: The patient should completely empty their bladder at a specific time (e.g., 8:00 AM) and discard this urine—this marks time zero. 1
Collection period: From that moment forward, collect every single void for the next 24 hours in the provided container. 1
End the collection: Just before the 24-hour period ends (e.g., 7:55 AM the next day), the patient should empty their bladder one final time and add this urine to the collection container—this completes the collection. 1
Timing considerations: The patient should try to delay the final voiding until just before the interval ends to ensure accuracy. 1
Pre-Collection Patient Instructions
Avoid vigorous exercise for 24 hours before and during collection, as physical activity causes transient elevation in protein and other analyte excretion. 1, 2
Women should avoid collection during menstruation due to potential blood contamination causing false elevations. 2
Treat any urinary tract infection first and retest after resolution, as symptomatic UTIs cause transient proteinuria elevation. 2
Maintain usual dietary habits unless specifically instructed otherwise by the physician, as the goal is to assess typical excretion patterns. 1
Collection Frequency Requirements
For patients who void infrequently (3 or fewer times per 24 hours): A 48-hour collection is recommended to avoid sampling errors and ensure adequate representation. 1
For all other patients: A standard 24-hour collection is sufficient. 1
In children: The collection period may be reduced to a minimum of 12 hours if supervised and the child voids frequently enough. 1
Verification of Collection Adequacy
Always measure 24-hour creatinine excretion simultaneously with the target analyte to verify completeness of collection—without this verification, the estimated daily excretion is often incorrect. 2
Studies demonstrate that more than 30% of 24-hour urine collections are incomplete and understate true excretion. 3
Expected 24-hour creatinine excretion can be estimated based on weight, sex, race, and age to identify incomplete collections. 3
If measured creatinine is substantially lower than expected, the collection is likely incomplete and should be repeated. 3
Volume Measurement and Sample Processing
Measure total volume accurately using a container with precision to within 50 mL per 2,000 mL. 1
Record the exact total volume in milliliters before taking any aliquots for laboratory analysis. 1
Mix the entire collection thoroughly by shaking or stirring before removing samples for testing. 1
Special Considerations for Specific Analytes
For calcium, magnesium, phosphorus, oxalate, and uric acid measurements (kidney stone evaluation): The collection container should contain acid preservative (typically 6N HCl to achieve pH 1.5), followed by alkalinization and heating protocols as needed. 4
For protein measurement: No special preservatives are typically required, though the specimen should be refrigerated during collection. 1
For aldosterone, sodium, and metanephrines: These can be measured simultaneously from the same 24-hour collection when clinically indicated. 5
Common Pitfalls to Avoid
Incomplete collection is the most common error: Missing even one void invalidates the entire collection. 3
Starting the collection with the first morning void instead of discarding it leads to inaccurate timing. 1
Failing to include the final void at the end of the 24-hour period results in undercollection. 1
Not measuring creatinine simultaneously prevents verification of collection adequacy. 2
Collecting during acute illness, marked hyperglycemia, marked hypertension, or heart failure can cause transient elevations that don't reflect baseline kidney function. 1
When 24-Hour Collection is Specifically Indicated
For patients with glomerular disease requiring initiation or intensification of immunosuppression, as 24-hour collection provides the most precise baseline measurement for treatment decisions. 5, 2
To confirm nephrotic syndrome (>3.5 g/day protein), as this diagnosis has critical implications for thromboprophylaxis management. 2
In patients with extremes of body habitus (cachexia, muscle atrophy, extreme obesity) where spot urine protein-to-creatinine ratio may be inaccurate due to abnormal creatinine excretion. 2
For simultaneous measurement of creatinine clearance in peritoneal dialysis patients or when GFR estimation equations are unreliable. 1, 5
Alternative to 24-Hour Collection
Spot urine protein-to-creatinine ratio has largely replaced 24-hour collections for routine screening and monitoring of proteinuria, as it eliminates collection difficulties and speeds decision-making. 2
However, spot ratios show more variability and are not ideal when precise quantification is needed for treatment decisions. 2