Urine sample contamination occurs when foreign substances, such as bacteria or other materials, mix with a urine specimen during or after collection, potentially interfering with laboratory test results. It is a common occurrence in healthcare settings, and while it can complicate diagnosis, it is often a preventable issue through proper collection techniques.
Common Causes of Contamination
Contamination often originates from external sources rather than from within the urinary tract. Bacteria from the skin around the perineum and the urethral opening are frequent culprits, entering the sample as urine passes through these areas during collection, especially if the genital area is not adequately cleansed. For female patients, vaginal secretions or menstrual blood can inadvertently mix with the urine, introducing additional cells or microorganisms not indicative of a urinary tract infection. Similarly, small amounts of fecal matter can contaminate the sample, as common fecal organisms like E. coli or enterococci can influence test results. Beyond biological contaminants, external substances such as soap residue or toilet water can also alter the sample’s composition.
Why a Clean Sample is Important
A clean urine sample is important for accurate diagnostic testing. Contaminants can lead to inaccurate test results, potentially causing false positives or obscuring true medical conditions. For instance, a sample showing the growth of several different types of bacteria often suggests contamination rather than a single causative agent.
Inaccurate results can lead healthcare providers down the wrong diagnostic path, resulting in misdiagnosis or the prescription of inappropriate treatments, such as unnecessary antibiotics. This is problematic given the global rise in antibiotic resistance.
A contaminated sample necessitates retesting, which delays accurate results and treatment. This also creates inconvenience for patients, requiring repeat visits and potentially incurring additional costs for follow-up tests.
Steps for Proper Urine Collection
Obtaining a “clean catch” midstream urine sample is a standardized method designed to minimize contamination and ensure accurate test results. First, wash hands thoroughly with soap and water. Open the sterile collection cup carefully, ensuring the inside of the cup or lid is not touched.
For female patients, separate the labia (the folds of skin around the urinary opening) with one hand. Use a provided cleansing wipe to wipe the area from front to back, cleaning the inner folds and around the urethral opening. Repeat this step with a second wipe.
For male patients, cleanse the head of the penis with a wipe. If uncircumcised, retract the foreskin and hold it back during collection.
Next, begin urinating a small amount into the toilet first. This initial flow flushes out any bacteria or debris from the urethra. Without stopping, position the collection cup into the urine stream to collect the middle portion. Fill the cup approximately halfway, avoiding contact between the cup and the body.
After collecting, the remaining urine can be passed into the toilet. Screw the lid on tightly, and submit the sample promptly to the laboratory. If transport is delayed, refrigerate the sample for up to 24 hours to prevent bacterial overgrowth.
What Happens After Contamination
If a urine sample is reported as contaminated, the most common next step is to provide another sample. Healthcare providers will often provide clearer, more detailed instructions for collection to help ensure the next sample is clean. This retesting is important to obtain an accurate picture of the patient’s urinary health.
A healthcare provider will review the results of a contaminated sample to determine if the presence of certain bacteria or cells might still suggest an underlying issue. For example, while mixed growth usually indicates contamination, it can sometimes be challenging to differentiate from a polymicrobial infection, which involves multiple types of bacteria. Based on clinical symptoms and retest results, further diagnostic steps or appropriate treatment can then be initiated.