Leukocyte esterase (LE) is an enzyme frequently tested as part of a routine urinalysis. Its presence in a urine sample is a significant finding that serves as an early indicator of inflammation or infection within the urinary tract. The test acts as a quick screening tool for conditions that cause an influx of white blood cells (leukocytes), the body’s primary immune defenders, into the urinary system. This marker gives healthcare providers an initial signal that the immune system is actively responding to something.
Biological Function of Leukocyte Esterase
The enzyme leukocyte esterase is produced and released primarily by neutrophils, the most common type of white blood cell in the body. Neutrophils are rapidly mobilized from the bloodstream to the site of an injury or infection as part of the innate immune response. Their role is to seek out and destroy foreign invaders, such as bacteria, through a process called phagocytosis.
The release of leukocyte esterase occurs when these immune cells become activated or undergo lysis (cell breakdown). The presence of LE in urine is not a measure of the enzyme itself, but rather an indirect biomarker indicating that neutrophils have been recruited to the urinary tract.
When white blood cells are present in the urinary tract to fight an infection, they shed the enzyme into the urine. This process results in pyuria, which is the term for having an abnormally high number of white blood cells in the urine. Detecting LE in a urine sample confirms the presence of these immune cells and their activity in the urinary system.
The Mechanics of Urinalysis Testing
The detection of leukocyte esterase is a standard component of chemical urinalysis, typically performed using a small, plastic reagent strip known as a dipstick. This convenient method allows for rapid testing in a clinical or laboratory setting. The dipstick contains a chemically treated pad that reacts with the LE enzyme.
When the strip is dipped into the urine sample, the enzyme, if present, hydrolyzes an acid ester compound on the pad. This initial reaction releases a specific alcohol, which then couples with a diazonium salt embedded in the pad. The result of this two-step chemical process is a visible color change on the pad, usually turning various shades of purple.
The intensity of the resulting color is generally proportional to the concentration of LE and the amount of white blood cells in the urine. Because the dipstick test is a screening method, a positive result often prompts a more detailed microscopic examination of the urine sediment. This microscopic review confirms the actual number of white blood cells present and can also identify bacteria, red blood cells, or other cellular elements.
Understanding Positive and Negative Results
A negative result for leukocyte esterase is considered normal and suggests no significant level of white blood cells or inflammation in the urinary tract. This finding indicates that the urine sample does not contain the enzyme in quantities large enough to trigger the chemical reaction on the dipstick. However, a negative result does not completely rule out an infection, as some infections may not produce a high concentration of LE, or the enzyme may be inactive due to high levels of protein or vitamin C in the urine.
A positive LE result, which can be reported as trace, small, moderate, or large, signifies the presence of pyuria (elevated white blood cells in the urine). A trace result suggests a low level of immune activity and may require correlation with other test factors and clinical symptoms before a diagnosis is made. Moderate or large positive results are a strong indication of an inflammatory response, most commonly due to infection.
A positive LE test is a screening result, not a definitive diagnosis of a specific condition. The result suggests the need for further investigation, such as a urine culture, to identify the exact type of bacteria causing the infection and determine the appropriate treatment. A positive result prompts the healthcare provider to look closer at the patient’s symptoms and other urinalysis components like nitrites.
Common Causes for Elevated Leukocyte Esterase
The most frequent cause of elevated leukocyte esterase is urinary tract infections (UTIs), including infections of the bladder (cystitis) and the urethra. When bacteria invade the urinary tract, the immune system floods the area with neutrophils, releasing LE. More serious kidney infections, known as pyelonephritis, also cause a strong positive result due to the inflammatory response in the upper urinary tract.
Other Causes of Elevated LE
Other conditions that can elevate LE levels include:
- Kidney stones, which cause irritation and inflammation as they pass through the tract.
- Inflammation of the kidneys (glomerulonephritis).
- Sexually transmitted infections like chlamydia or trichomoniasis.
- Contamination of the urine sample, such as from vaginal discharge or poor collection technique.
Sometimes, high levels of white blood cells are found in the urine without any detectable bacteria, a condition known as sterile pyuria. This can be caused by certain medications, autoimmune diseases like systemic lupus erythematosus, or infections that do not grow on standard culture, such as tuberculosis. The presence of LE, even without bacteria, signals an underlying inflammatory process that requires medical attention and follow-up testing.