Urine samples are a common tool in medical testing for diagnosing conditions and monitoring health. The accuracy of these tests relies heavily on the quality and integrity of the collected specimen. Because of this requirement for a pristine sample, combining two separate voids is generally not allowed; a single, continuous collection is necessary for most standard diagnostic tests.
The Necessity of a Single Continuous Sample
Combining two separate voids, even if they occur minutes apart, will compromise the reliability of the laboratory results. This practice immediately alters the urine’s concentration, which is a fundamental measurement in urinalysis. The specific gravity and concentration of chemical markers are highly dependent on the hydration status and the time the urine has spent in the bladder.
Mixing samples creates an averaged, artificial reading that does not accurately reflect the body’s condition, potentially leading to misdiagnosis. For example, the concentration of substances like creatinine, often used to standardize results, can fluctuate significantly between separate voids. If the test requires specific timing, such as a first-morning void, combining it with a later collection would invalidate that parameter.
Executing the Midstream Clean Catch Technique
For many diagnostic tests, especially those looking for infection, the required collection method is the “midstream clean catch” technique. This method prevents external bacteria from contaminating the specimen. The goal is to obtain a sample accurately representing the urine flowing through the urethra and bladder.
The process begins by cleaning the genital area with the provided towelettes, wiping from front to back to remove surface flora. After cleaning, the individual starts to urinate directly into the toilet, allowing the initial flow to wash out any remaining bacteria from the urethra. The collection cup is then carefully introduced into the stream to capture the middle portion of the void. Finally, the cup is removed before the flow stops, and the remaining urine is passed into the toilet, ensuring only the clean middle portion is tested.
Addressing Insufficient Volume or Contamination
A common issue is failing to collect enough urine, often labeled as “Quantity Not Sufficient” or QNS. Laboratories generally require a minimum volume, which can range from 10 to 20 milliliters for a full urinalysis. If the volume is too small, the lab may not have enough liquid to perform all the necessary tests, or the concentration of required substances may be too low.
If a collection error occurs, such as contamination from accidentally touching the inside of the cup or having an insufficient volume, the sample is compromised. In such cases, the protocol is to discard the sample and attempt a re-collection, after immediately informing the lab technician or nurse. Depending on the test and hydration level, this may require waiting 30 to 60 minutes to allow the bladder to refill, often with instructions to drink water to facilitate the next void.