A positive result on a urinary tract infection (UTI) test suggests the presence of pathogenic bacteria, but this finding is not always accurate. A false positive occurs when the test indicates an infection, yet a subsequent culture reveals no significant bacterial growth. Understanding the factors that interfere with the test or contaminate the sample is important for accurate diagnosis and preventing unnecessary antibiotic use.
How Standard UTI Tests Work
The standard initial screening for a UTI relies on a simple urine dipstick test, which measures the concentration of several substances. Two primary indicators for a bacterial infection are Leukocyte Esterase (LE) and Nitrites. LE is an enzyme released by white blood cells (leukocytes) present during inflammation or infection.
Nitrites are not normally found in urine; their presence suggests that certain bacteria, primarily Enterobacteriaceae like E. coli, have converted dietary nitrates into nitrites. The dipstick also screens for blood and protein. While a positive result for both LE and Nitrites suggests a UTI, a positive for only one can be a consequence of non-infectious factors.
Errors During Sample Collection
The most common source of a false positive is contamination during sample collection. Healthcare providers request a “clean catch” midstream sample, but improper technique frequently compromises the specimen. If the genital area is not adequately cleaned, bacteria and white blood cells from the surrounding skin or vaginal discharge can enter the cup.
Contamination from vaginal flora or epithelial cells frequently causes false Leukocyte Esterase (LE) and bacterial readings in women. If the sample is not processed promptly, bacteria naturally present in the urine multiply rapidly at room temperature. This overgrowth, which does not reflect the concentration in the bladder, can lead to a false positive nitrite result or a high bacterial count on a culture.
Chemical Interference from Medications
Certain substances ingested by the patient can chemically interfere with the color-changing reagents on the dipstick, mimicking a positive result. The over-the-counter urinary pain reliever Phenazopyridine (Azo) is a well-known culprit. This medication contains a bright orange-red dye that physically stains the dipstick, leading to a false-positive reading for multiple components, particularly blood.
High doses of certain vitamins can also interfere with the test’s chemistry. For example, Ascorbic Acid (Vitamin C) acts as a strong reducing agent that can prevent the dipstick from correctly detecting nitrites or blood. This interference leads to a false negative result.
Non-Infectious Physiological Causes
Inflammation not caused by a bacterial infection can produce sterile pyuria, where white blood cells are present without significant bacteriuria. A recent course of antibiotics might have cleared a mild infection, but white blood cells may still be shedding for up to two weeks, causing a transient false-positive Leukocyte Esterase (LE) result.
Kidney stones or recent trauma can cause micro-bleeding and inflammation, leading to positive readings for blood and LE without infection. Chronic inflammatory conditions, such as interstitial cystitis or certain kidney diseases, also cause chronic pyuria and proteinuria.