Specimen Requirements and Procedure
Specific instructions for collecting a 24-hour urine sample may vary by laboratory. Typically, the patient’s first voided morning urine sample is discarded. Subsequent urine over the next 24 hours, including the first void of the next morning, is collected in containers that the laboratory provides. For example, if the urine specimen collection begins at 9 am, the patient would void into the toilet at 9 am, collect all urine in the specimen bottle for the next 24 hours, and include the 9 am void the following day. The sample should be refrigerated throughout the collection period.
A preservative solution is added to the urine collection to stabilize the sample for later analysis. After the full 24-hour collection, the total volume is recorded, and a representative sample is submitted to the laboratory for analysis. Proper patient education on collection procedures is crucial for ensuring the accuracy of the 24-hour urine sample.[10] The evaluation often includes serum samples for creatinine, urea, calcium, potassium, uric acid, and phosphorus. Patients are typically advised to follow their usual diet and activities during the collection period.[11][12]
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After analysis, a detailed report of the results is provided to the ordering clinician. Collecting a 24-hour urine sample can be challenging and inconvenient for some patients. As mentioned, if a full 24-hour collection is not feasible, a shorter collection can be used, provided the instructions are followed carefully and the time is precisely recorded. Accurate results reliably identify urinary chemistry risk factors for calculus formation and can aid in diagnosing other medical conditions, as spot urine chemistry is often insufficient. For kidney stone prevention, a properly collected 24-hour urine analysis is as valuable as analyzing the chemical composition of the stone itself. Please see StatPearls’ companion resource, “24-Hour Urine Testing for Nephrolithiasis: Interpretation and Treatment Guidelines,” for more information.
For pheochromocytoma (metanephrine) urine testing, it is recommended that patients avoid food, caffeine, smoking, and strenuous exercise for 8 to 12 hours before sample collection. Many medications can interfere with metanephrine levels, leading to false positives. Drugs such as acetaminophen, tricyclic antidepressants (eg, amitriptyline), serotonin-norepinephrine reuptake inhibitors (eg, buspirone), alpha-blockers (eg, tamsulosin), haloperidol, labetalol, levodopa, lamotrigine, aripiprazole, and possibly selective serotonin reuptake inhibitors can elevate plasma and urine metanephrines. Notably, it is estimated that drug interference causes up to 20% of false-positive results, with tricyclic antidepressants and the alpha-blocker phenoxybenzamine accounting for up to 45% of these cases. Therefore, when possible, these medications should be discontinued 10 to 14 days before testing.[8][13]
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