Urinalysis is a common diagnostic procedure where a urine sample is examined to check for signs of various health conditions, including kidney issues, diabetes, and urinary tract infections (UTIs). The reliability of a urinalysis hinges entirely on the quality of the specimen provided. When a sample is compromised by outside material, it is deemed contaminated, which can render the test results inconclusive or misleading.
Defining Contamination in Urinalysis
Contamination in a clinical setting refers to the introduction of foreign substances into the urine sample after it has left the bladder. Since urine within the bladder is generally sterile, the presence of external matter suggests an issue with the collection process, not an internal medical problem. This differs from a true infection, where a single type of pathogen multiplies within the urinary tract.
Laboratory staff identify a contaminated sample through specific microscopic findings. A high concentration of squamous epithelial cells, which are shed from the outer skin and genital area, is a primary indicator of contamination. For instance, a count exceeding 15 to 20 squamous cells per high-power field (HPF) often signals a compromised specimen.
Another indicator is a urine culture result showing “mixed flora” or polymicrobial growth. This means the sample contains multiple types of bacteria, usually at low concentrations, characteristic of bacteria transferred from the skin or genital region during collection. In contrast, a true, uncomplicated UTI is caused by a high concentration of a single bacterial species, such as E. coli.
Common Sources of Sample Contamination
The sources of contamination are almost always external, stemming from patient actions or the collection environment. The most frequent cause is insufficient cleansing of the genital area before voiding the sample. For female patients, the proximity of the urethra to the vaginal and rectal areas makes the sample particularly susceptible to contamination from surrounding flora, including vaginal discharge or fecal matter.
Bacteria that naturally reside on the skin, known as epidermal flora, can easily enter the container if the initial stream of urine is not discarded. Contact between the urine stream or the collection cup and the skin, pubic hair, or clothing introduces these external organisms. Additionally, accidental inclusion of menstrual blood can interfere with the results, mimicking a condition called hematuria, or blood in the urine.
Contamination can occur after collection due to non-sterile handling or delayed processing. Using a non-sterile collection cup, touching the inside of the sterile cup, or allowing the sample to remain at room temperature for too long encourages rapid bacterial growth. This late-stage growth can artificially inflate the bacterial count, making a non-infected sample appear to have a clinically significant infection.
Interpreting Contaminated Results and Next Steps
The presence of contamination complicates diagnosis, often leading to inconclusive results. A contaminated sample complicates the interpretation of findings, as external bacteria and white blood cells can falsely suggest a mild urinary tract infection. This can result in a false-positive diagnosis, potentially leading to unnecessary antibiotic treatment and contributing to antibiotic resistance.
When a laboratory identifies a compromised specimen, the report often includes comments like “mixed flora,” “contaminated specimen,” or notes an abundance of squamous epithelial cells. The healthcare provider informs the patient that the results are not reliable for a definitive diagnosis. Since the test integrity is compromised, any potential underlying condition cannot be accurately assessed.
The immediate next step is almost always a recollection of the sample. This retest is required to obtain an accurate diagnosis and avoid misinterpretation. The healthcare provider emphasizes the need for a more stringent collection process, often by providing detailed instructions for a clean-catch technique.
Essential Techniques for Proper Sample Collection
To prevent contamination, patients must follow the midstream clean-catch technique precisely, as this procedure minimizes the introduction of external microorganisms. The process begins with thorough handwashing before handling the collection materials. The sterile specimen cup should be opened carefully, ensuring the inside rim and interior are not touched.
For female patients, the steps involve separating the skin folds around the urinary opening and wiping the area from front to back using the provided antiseptic wipes. Male patients should wipe the head of the penis with the antiseptic wipe. After cleansing, the patient should begin to urinate into the toilet to flush out any bacteria that reside at the opening of the urethra.
After allowing a small amount of urine to pass into the toilet, the sterile cup is positioned to collect the “midstream” portion of the urine. The flow should be stopped before the stream is completely finished, and the cup should be capped immediately and securely. The collected sample must be delivered to the laboratory promptly or refrigerated if a delay is unavoidable, to inhibit bacterial multiplication.