A urinary tract infection (UTI) occurs when microbes, most often bacteria, enter and colonize any part of the urinary system. Testing is performed when a person has symptoms suggesting an infection, such as painful or frequent urination, to confirm the presence of bacteria and guide treatment. Determining a positive result requires evaluating markers across two primary methods: rapid dipstick urinalysis and the more definitive laboratory urine culture. Understanding a positive result involves recognizing both the immediate color changes from screening tests and the specific numerical thresholds from lab reports. This analysis helps distinguish a true infection requiring treatment from other conditions or simple contamination.
Visual Indicators from Rapid Dipstick Tests
The quickest way to screen for a UTI is through a rapid dipstick test, which provides near-immediate results by detecting two main chemical markers in the urine. These strips rely on a chemical reaction that produces a visual color change to indicate a positive finding. The two primary indicators are Leukocyte Esterase (LE) and Nitrites.
A positive result for Leukocyte Esterase suggests the presence of white blood cells (leukocytes) in the urine, a condition known as pyuria. The LE test pad will change to a color darker than the negative control, often appearing as a shade of purple or darker cream, signifying the body’s immune response to inflammation or infection. The intensity of this color change corresponds to the concentration of white blood cells detected.
Nitrites are a highly specific indicator of certain types of bacteria that cause UTIs. A positive nitrite result appears as a change to a darker color, typically pink or purple, on the corresponding test pad. This reaction detects nitrites, which are produced by bacteria converting natural urinary nitrates. A positive result on both the LE and Nitrite pads is a strong visual indicator that a bacterial UTI is present, prompting further medical evaluation.
Interpreting Quantitative Lab Culture Results
While the dipstick offers a quick screen, a definitive diagnosis comes from a quantitative urine culture performed in a laboratory. This process involves incubating a urine sample to allow any present bacteria to multiply and form visible colonies. The lab report provides a positive result by identifying the specific type of bacteria and measuring its concentration.
The bacterial concentration is measured in Colony Forming Units per milliliter (CFU/mL) of urine. For a clean-catch urine sample from a symptomatic patient, a positive culture is generally defined by the growth of a single type of bacteria at a concentration of 10^5 CFU/mL. This high threshold traditionally distinguishes a true infection from minor contamination that can occur during sample collection.
The laboratory report will also identify the specific organism causing the infection, with Escherichia coli (E. coli) being the most common culprit in up to 90% of uncomplicated UTIs. A positive result confirms the presence of an infectious pathogen. In some cases, such as with catheter-collected samples or in highly symptomatic patients, a significantly lower count, like 10^3 CFU/mL, may still be considered positive, especially if white blood cells are also present. Following identification, the lab performs susceptibility testing to determine which antibiotics will be most effective in treating the confirmed infection.
Understanding Specific Positive Markers
Leukocyte Esterase (LE) and Nitrites each have a distinct biological significance. LE is an enzyme released by neutrophils, a type of white blood cell, as they actively fight off an infection. Its presence signals an inflammatory response in the urinary tract.
A positive LE result indicates pyuria (white blood cells in the urine), but it does not specify the cause, as inflammation can also be due to other conditions. Conversely, Nitrites are a direct chemical byproduct of bacterial metabolism. Certain gram-negative bacteria, like E. coli, possess the enzyme necessary to convert dietary nitrates into nitrites. Because nitrites are generated only by specific bacteria, a positive nitrite test is highly specific, strongly indicating a bacterial presence.
Interpreting mixed results is important. A positive LE result combined with a negative nitrite result may occur if the infection is caused by a bacteria that does not convert nitrate, such as Enterococcus. This can also happen if the urine did not remain in the bladder long enough for the conversion to take place. Conversely, a positive nitrite with a negative LE can sometimes indicate an early infection or that the white blood cells have broken down. However, both positive markers together offer the highest predictive value for a confirmed UTI.