Nitrites are not normally found in healthy urine, so their presence strongly indicates a potential urinary tract infection (UTI). This often suggests an active infection, typically caused by Gram-negative bacteria like Escherichia coli. A “false positive” occurs when the test strip shows nitrites are present, but a true urinary tract infection does not exist. Understanding the non-infectious factors that trigger this misleading result is essential for accurate diagnosis and avoiding unnecessary treatment.
Understanding the Nitrite Test Principle
The detection of nitrites relies on a specific biological conversion process involving certain bacteria. Urine naturally contains harmless nitrates, primarily derived from the diet. Bacteria that commonly cause UTIs, such as E. coli, possess the enzyme nitrate reductase. This enzyme converts nitrates into reactive nitrites. The nitrite test strip uses a chemical reaction that changes color when it encounters these nitrites, signaling the presence of nitrate-reducing bacteria.
Errors in Sample Collection and Storage
One of the most frequent causes of a false positive result is bacterial activity that happens after the urine sample has been collected. Even a clean-catch sample contains a minimal amount of bacteria from the genital area or the environment. If the urine specimen remains at room temperature for an extended time before it is tested, these low levels of bacteria can multiply rapidly. This accelerated bacterial growth then converts the nitrates in the urine to nitrites outside the body in the collection container. Research suggests that a significant number of samples can show a false positive nitrite result if testing is delayed beyond one hour.
Delayed testing allows even small amounts of harmless contamination to produce enough nitrite to trigger a positive result, even without a clinical infection. Proper procedure requires the sample to be tested immediately or refrigerated quickly to inhibit this bacterial overgrowth. Failure to use a sterile collection method, like a midstream clean-catch technique, also introduces a higher initial load of contaminating bacteria, increasing the risk of post-collection conversion.
Chemical Interference and Test Strip Integrity
False positive results can be caused by chemical interference within the urine or issues with the testing materials. Certain medications are known to react non-specifically with the chemicals on the dipstick. A prime example is phenazopyridine, an over-the-counter medication used to relieve urinary pain, which imparts a vivid orange-red color to the urine. This intense pigmentation can visually interfere with the test strip’s colorimetric reaction, leading to a false interpretation of a positive result.
The integrity of the testing equipment is another factor. If the test strips are expired or improperly stored, the embedded reagents may degrade or become contaminated. Exposure to excessive moisture or air can destabilize the chemicals, causing them to react inaccurately and produce a misleading positive color change. High concentrations of certain substances in the urine can also react unexpectedly with the test reagents, creating a false signal distinct from bacterial activity.