Urine esterase is frequently checked during a routine urine test, known as a urinalysis. The term refers specifically to the enzyme Leukocyte Esterase (LE), which is not normally found in detectable amounts in healthy urine. When the test detects this enzyme, it suggests that white blood cells are present in the urinary tract, a condition medically termed pyuria. A positive result signals the possibility of inflammation or an active infection within the urinary system that requires further evaluation.
The Enzyme’s Origin and Role
Leukocyte Esterase originates from white blood cells (leukocytes), which are a fundamental component of the body’s defense system. The enzyme is produced primarily by neutrophils, a type of white blood cell that rushes to sites of infection or injury. Neutrophils release LE to break down harmful bacteria and cellular debris. Finding this enzyme in a urine sample shows that these defense cells have been mobilized to the urinary tract. The presence of LE serves as an indirect marker, indicating white blood cells are involved in an inflammatory process somewhere from the kidneys to the urethra.
The Dipstick Test Procedure
Detection of this enzyme is most commonly performed using a specialized chemical strip known as a reagent dipstick. This rapid screening method is a quick and cost-effective way to check for various substances in the urine, including Leukocyte Esterase. The test area on the dipstick contains a chemical reagent designed to react specifically with the enzyme. When the strip is dipped into a urine sample containing LE, the enzyme hydrolyzes an acid ester on the pad, releasing an alcohol. This alcohol reacts with a diazonium salt, causing the test area to change color, typically turning purple. The intensity of the color change corresponds to the amount of enzyme present, with results often reported as “trace,” “small,” “moderate,” or “large.”
Interpreting Positive Results
A positive result for Leukocyte Esterase strongly suggests pyuria, meaning an abnormally high concentration of white blood cells in the urine. This finding is associated with a Urinary Tract Infection (UTI), but it can also indicate other inflammatory conditions. These may include kidney stones, inflammation of the kidneys (pyelonephritis), or sexually transmitted infections affecting the urinary tract.
Because a positive LE test is a screening tool, healthcare providers often correlate this result with other urinalysis findings, such as the presence of nitrites. Nitrites are a separate marker indicating the presence of certain types of bacteria which convert dietary nitrates into nitrites.
The most definitive interpretation comes from examining the microscopic elements of the urine sample. A positive LE result is often confirmed by counting the actual white blood cells under a microscope. A count of five or more white blood cells per high-power field typically confirms pyuria.
If both the LE and the nitrite tests are positive, the likelihood of a bacterial UTI is significantly increased. However, a positive LE result alone necessitates a follow-up urine culture. The culture is the gold standard for identifying the exact type of bacteria causing the infection and determining the most effective antibiotic treatment.
Factors Affecting Accuracy
While the dipstick test is a reliable screening tool, its results can sometimes be misleading due to various interfering factors. One common cause of a false-positive result is contamination of the sample, such as when vaginal discharge or blood mixes into the urine specimen. Certain oxidizing agents, often found in genital cleansing products, can also cause the dipstick to change color incorrectly.
Conversely, a false-negative result can occur when the test fails to detect the enzyme even if an infection is present. This happens if the urine is highly diluted, causing white blood cells to break down and release the enzyme at a level too low for detection. High concentrations of substances like Vitamin C (ascorbic acid) or elevated levels of protein and glucose can chemically interfere with the color reaction, leading to a negative reading despite an underlying infection.