“Probable contaminants” in a urine culture means the sample likely contains bacteria or other microorganisms not from an actual urinary tract infection (UTI). This common finding suggests the sample was compromised by external factors during collection or handling, usually microbes from the skin or genital area.
Contamination Versus Actual Infection
Distinguishing between a contaminated urine sample and a true urinary tract infection involves analyzing several factors in the laboratory. A genuine UTI typically shows a significant presence of a single type of bacteria, along with signs of inflammation. In contrast, a contaminated sample often contains multiple types of bacteria, or a mix of bacteria commonly found on the skin, without one dominant strain.
Laboratory professionals assess the number of bacteria, referred to as colony-forming units (CFU) per milliliter, and the types of organisms present. For a clean-catch urine sample, a count of 100,000 CFU/mL or more of a single bacterial type typically suggests an infection. However, lower counts, such as 1,000 to 100,000 CFU/mL, can still indicate an infection, especially if symptoms are present or the sample was collected via a catheter. If a culture shows several types of bacteria without a clear dominant one, it is often interpreted as contamination.
The presence of inflammatory markers also helps differentiate between contamination and infection. A urinalysis, often performed alongside a urine culture, looks for white blood cells (pyuria) and nitrites. Nitrites suggest the presence of bacteria that convert nitrates, while white blood cells indicate the body’s immune response to an infection. If a urine culture is positive for bacteria but there are no symptoms or signs of inflammation like pyuria, it might indicate asymptomatic bacteriuria or a contaminated sample.
Sources of Contamination
Urine samples can become contaminated by microorganisms from the surrounding environment during collection. The most common source is normal bacteria on the skin around the genital and perineal areas, such as those from the vagina, perineum, and epidermis, which can inadvertently enter the collection cup.
Improper cleaning of the genital area before providing the sample is a frequent cause. If not thoroughly cleansed, skin flora can be washed into the urine stream. Similarly, if the collection container is not sterile or its inner surface is touched, environmental bacteria or hand bacteria can be introduced. Delay in transport or improper storage (e.g., room temperature) can also lead to contaminants multiplying.
Next Steps After a Contaminated Result
When a urine culture indicates “probable contaminants,” healthcare providers typically review the result in the context of the patient’s symptoms. If the patient has no symptoms of a urinary tract infection, such as painful urination, frequent urges, or fever, antibiotic treatment is usually not initiated. Treating asymptomatic bacteriuria, which is bacteria in the urine without symptoms, is generally not recommended as it can lead to antibiotic resistance and does not prevent future infections.
A common next step is to repeat the urine test, ensuring proper clean-catch collection instructions are followed for a more accurate sample. If symptoms persist despite a contaminated or negative culture, the healthcare provider may consider other potential causes for the patient’s discomfort, as various conditions can mimic UTI symptoms. Patients should discuss their results and any ongoing symptoms with their provider to determine the appropriate course of action.
Preventing Contamination
Collecting a “clean catch” urine sample is essential for accurate test results. This method obtains a midstream urine sample, meaning the initial flow is discarded to flush out bacteria from the urethra and surrounding skin.
To perform a clean catch:
- Wash hands thoroughly with soap and water.
- Females: Separate skin folds and clean the area front to back with provided wipes.
- Males: Clean the tip of the penis, retracting foreskin if uncircumcised.
- Begin to urinate into the toilet for a few seconds.
- Place the sterile collection cup into the ongoing stream to collect the middle portion.
- Avoid touching the inside of the cup or lid.
- Finish urinating into the toilet.
- Securely cap the container and deliver the sample to the laboratory as soon as possible.
- If delayed, refrigerate the sample for no more than 24 hours.