Leukocyte esterase (LE) is a specific enzyme produced by white blood cells, the body’s primary immune defense cells. Detecting LE is a simple, non-invasive method used to quickly indicate that an immune response is underway. The measurement of this enzyme is a widely accepted part of routine laboratory work to assess for inflammation or infection within the body.
What is Leukocyte Esterase?
Leukocyte esterase is a type of protein that functions as an esterase enzyme. It is primarily contained within the granules of a specific type of white blood cell known as a neutrophil. Neutrophils are the most abundant kind of white blood cell and are typically the first responders to bacterial infections and acute inflammation.
When white blood cells, particularly neutrophils, are activated to fight an infection or are present due to inflammation, they release this enzyme. The presence of LE in a bodily fluid is therefore an indirect, yet reliable, indicator of active white blood cells. Its release acts as a signal that the immune system is actively engaged at that location.
Measuring Leukocyte Esterase in Diagnostics
The detection of leukocyte esterase is most commonly applied in routine urinalysis. This simple procedure uses a reagent strip, or dipstick, which contains a specialized chemical pad. The test screens for pyuria, the presence of an abnormally high number of white blood cells in the urine, suggesting inflammation or infection in the urinary tract.
The dipstick test detects esterase activity through a specific chemical reaction. If LE is present, it hydrolyzes a chemical substrate on the pad to release an alcohol, which then reacts with a diazonium salt to produce a distinct color change, typically purple. The intensity of this color change is proportional to the amount of enzyme present, providing a semi-quantitative measure. This method is a cost-effective and rapid screening tool, with results read visually or by an instrument within two minutes. A normal urine sample should result in a negative or trace finding.
Interpreting Positive Findings
A positive leukocyte esterase result strongly indicates pyuria, meaning elevated levels of white blood cells are in the urine. This finding suggests an inflammatory process is active within the urinary system (kidneys, ureters, bladder, or urethra). The most frequent reason for a positive LE test is a bacterial urinary tract infection (UTI).
A positive result signifies that the immune system has sent neutrophils to the urinary tract to combat a potential pathogen. While highly suggestive of a UTI, LE is not a definitive diagnosis alone. Other inflammatory conditions, such as kidney stones, bladder inflammation, or urethral inflammation, can also cause a positive result. More serious conditions, like kidney inflammation (nephritis) or certain sexually transmitted infections, may also be potential causes.
To confirm the presence of an infection and identify the specific bacteria, the healthcare provider typically orders follow-up tests. The most common follow-up is a urine culture, which attempts to grow and identify the specific microorganism. Microscopic analysis of the urine sediment is also performed to count the number of white blood cells and look for bacteria. These additional tests help distinguish between a simple inflammatory process and an active bacterial infection, guiding treatment decisions.
Factors Influencing Test Accuracy
Although the LE test is an excellent screening tool, its results can be influenced by various factors, leading to potential inaccuracies.
False-Negative Results
One limitation is that the test primarily detects esterase from granulocytic leukocytes; if the white blood cells present are mainly lymphocytes, the result may be falsely negative. Highly diluted urine can also cause white blood cells to burst (lyse) before testing, which can decrease the amount of detectable enzyme and result in a false-negative finding.
High concentrations of glucose (such as in uncontrolled diabetes) or excessive protein levels can reduce the sensitivity of the test, potentially leading to a false-negative result. Taking large doses of ascorbic acid (Vitamin C) is also known to interfere with the chemical reaction, often causing a false-negative outcome.
False-Positive Results
False-positive results can occur if the urine sample is contaminated with vaginal discharge or secretions that naturally contain white blood cells. Certain medications, such as some antibiotics, can also cause a false-positive reaction.