From the Guidelines
To collect a 24-hour urine sample, patients should empty their bladder in the toilet at the start, discard this initial urine, and then collect all subsequent urine in a special container for the next 24 hours, ending with a final voiding into the container at the end of the collection period, as recommended by guidelines 1. The process begins with the patient emptying their bladder and discarding the urine, then starting the collection period.
- The patient should collect every urination during the 24-hour period in the provided container.
- The container may contain a preservative to maintain the integrity of the sample.
- It is crucial to store the container in a cool place, such as the refrigerator, between uses to prevent degradation of the sample.
- At the end of the 24 hours, the patient should urinate one final time into the container to complete the collection.
- The sample should be kept refrigerated until it can be delivered to the laboratory, typically within a short timeframe after completion. This method is supported by clinical practice guidelines for ensuring the completeness and accuracy of urine collection, especially for patients who void frequently 1.
- For patients who make little urine and void infrequently, a 48-hour collection may be more appropriate, as it can provide a more comprehensive picture of their urinary excretion patterns.
- The guidelines emphasize the importance of collecting urine over a sufficient period to avoid sampling errors and ensure reliable test results.
From the Research
Collecting 24-Hour Urine
To collect a 24-hour urine sample, the following steps can be taken:
- Start by drinking enough fluid to urinate at least 2 L daily during the collection period 2
- A 24-hour urine collection can be performed to establish baseline levels of multiple urinary parameters before taking any medication or treatment 2
- The collection period typically starts after the first urination of the day and ends with the first urination of the next day
Ensuring Complete Collection
Methods to assess complete collection of 24-hour urine include:
- Para-aminobenzoic acid (PABA) recovery, which is considered the referent method 3
- Creatinine criteria, which has a sensitivity and specificity for identifying incomplete collection of 6% to 63% and 57% to 99.7%, respectively 3
- A creatinine index <0.7 is the most sensitive method for removing incomplete collections 3
Importance of Accurate Collection
Accurate 24-hour urine collection is crucial for:
- Estimating sodium intake, which is associated with cardiovascular risk 4
- Assessing the effectiveness of treatments, such as those for urolithiasis 5
- Evaluating the relationship between sodium and potassium excretion and cardiovascular disease 4
Challenges and Limitations
Challenges and limitations of 24-hour urine collection include: