Urine tests are fundamental in evaluating health, especially concerning urinary tract conditions. Among the markers tested, leukocyte esterase plays a critical role in identifying the presence of immune responses.
This article provides a comprehensive overview of leukocyte esterase, explaining its significance in clinical practice and offering a guide for interpreting test results.
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What Is Leukocyte Esterase?
Leukocyte esterase (LE) is an enzyme released by activated neutrophils, a type of white blood cell that primarily defends the body against bacterial infections. Neutrophils fight infection by rapidly migrating to the site of injury or infection, where they engulf and destroy pathogens, release antimicrobial substances, generate reactive oxygen species, and signal other immune cells to amplify the inflammatory response.
Purpose and Importance of the Leukocyte Esterase Test
A positive leukocyte esterase test indicates the presence of pyuria – a condition in which there are high levels of white blood cells (leukocytes) or pus in the urine. When infectious pathogens invade the body, the immune system signals white blood cells to migrate to the site of the infection to fight it. A positive test often signals the need for further investigation to identify the type of infection.
How the Test Is Performed
A clean-catch urine sample is required to measure LE. Instruct the patient to collect a urine sample with the clean-catch method, following these steps:
- Wash hands thoroughly with soap and water.
- Clean the genital area with a provided antiseptic wipe. Females should clean the labial folds from front to back with one wipe and the urethral opening with a second wipe. Males should clean the tip of the penis, retracting the foreskin if uncircumcised.
- Begin urinating into the toilet to flush out contaminants.
- Midstream, collect urine in a sterile container without touching the inside of the container.
- Finish urinating into the toilet and securely seal the container.
To perform the LE test:
- Remove a test strip from its packaging. To prevent contamination, avoid contact with the reactive pads on the strip.
- Fully immerse the reagent end of the dipstick into the urine sample for 1-2 seconds.
- Remove the dipstick, and gently tap or blot the side on absorbent paper to remove excess urine.
- Allow the strip to develop for the time specified by the manufacturer.
- Compare the color of the leukocyte esterase pad on the strip to the color chart provided on the test packaging. The color of the dipstick pad indicates if leukocytes are present in the urine.
Understanding Leukocyte Esterase Test Results
The LE urine test screens for infection or inflammation of the urinary system, which consists of the urethra, bladder, ureters, and kidneys.
Test results are categorized as follows:
- Negative: No color change, indicating no detectable esterase activity or white blood cells.
- Trace: A faint color change, suggesting minimal esterase activity, potentially from a mild or early-stage infection.
- Positive: A distinct color change, with levels further graded as:
- Small ('+')
- Moderate ('++')
- Large ('+++')
Diagnostic Role in UTIs and Other Infections
Most commonly, a positive LE test result is caused by a urinary tract infection (UTI). To diagnose a UTI, doctors will interpret the lab result in conjunction with patient symptoms and other laboratory findings. Findings diagnostic for a UTI include:
- Classic lower urinary tract symptoms:
- Painful urination
- Frequent voiding of small volumes
- Urinary urgency
- Blood in urine
- Discomfort above the pubic bone
- Urine studies:
- Positive nitrites and leukocyte esterase
- White blood cell count ≥ 10 white blood cells/mcL in urine
- Positive urine culture
However, elevated levels may indicate other conditions related to infection or inflammation in the kidneys or other surrounding organs, such as (13, 18):
- Kidney stones
- Kidney infection
- Glomerulonephritis (a type of kidney disease)
- Sexually transmitted infections
- Tuberculosis
- Viral infections
- Bladder tumors
- Autoimmune disease
- Balanitis (inflammation of the head of the penis)
- Prostatitis
Limitations of the Leukocyte Esterase Test
While highly useful, the test is not infallible. The leukocyte esterase test is not 100% sensitive and specific, meaning the test results can be either falsely positive or falsely negative.
False positives can occur due to:
- Sample contamination
- Recent exercise
- The use of corticosteroids and cylcophosphamide
False negatives can occur due to:
- Elevated specific gravity, often due to dehydration or kidney disease
- Elevated glucose (sugar) in urine, typical in patients with uncontrolled diabetes
- Ketones in urine, caused by fasting, low-carbohydrate diets, intense exercise, and diabetes
- Protein in urine, caused by dehydration, intense exercise, and kidney disease
- The use of cephalexin, nitrofurantoin, tetracycline, gentamicin, and vitamin C
When to Seek Medical Advice Based on Test Results
Patients should always follow up with their doctor if they have an elevated LE urine test. In most cases, this indicates an uncomplicated urinary tract infection, which can be easily treated with antibiotics or natural alternative therapies.
Certain symptoms indicate a more complicated infection that has traveled to the kidneys, which requires more intensive therapy to prevent serious complications like sepsis or kidney damage. Talk to a doctor immediately if you have any of these symptoms:
- Back or side pain
- Fever
- Shaking and chills
- Nausea or vomiting
Furthermore, patients and doctors should consider additional testing and imaging to follow up on negative results in symptomatic patients. A typical diagnostic evaluation of genitourinary symptoms of unknown origin includes:
- Renal function tests
- Serum electrolytes
- Urine culture and sensitivity
- Wet mount and potassium hydroxide (KOH) microscopy or a vaginal pathogens DNA probe
- Nucleic acid amplification test for Neisseria gonorrhoeae (gonorrhea) and Chlamydia trachomatis (chlamydia)
- Ultrasonography
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Key Takeaways
- The leukocyte esterase urine test is a simple and non-invasive test that can help diagnose urinary tract infections easily.
- It is included as part of a urinary dipstick test and often ordered during the first-line evaluation of UTI symptoms, such as painful and frequent urination, to confirm a clinical diagnosis.
- Positive urinary nitrites and elevated white blood cell counts (pyuria) support the diagnosis of a UTI.
- Because false positive and false negative test results can occur, it is important to interpret this test in conjunction with clinical symptoms and additional pertinent tests.
Frequently Asked Questions
What Does a Positive Leukocyte Esterase Test Mean?
A positive leukocyte esterase test indicates that white blood cells are present in the urine, suggesting infection or inflammation of the urinary tract. Most often, a urinary tract infection causes a positive LE test.
Can Leukocyte Esterase Levels Return to Normal Without Treatment?
Elevated leukocyte esterase levels can normalize on their own if caused by non-infectious factors. For example, vigorous exercise may temporarily increase leukocyte esterase, but levels typically return to normal once the physical stress is resolved.
What Should I Do If My Leukocyte Esterase Test Is Positive But I Have No Symptoms?
Follow up with your healthcare provider to rule out asymptomatic infections or other inflammatory conditions that could be causing your leukocyte esterase to be high.
How Often Should the Test Be Repeated if Results Are Ambiguous?
If the initial LE test results are unclear, repeat the test after ensuring proper sample handling and timing per the manufacturer's instructions. Your doctor may also recommend ordering alternative diagnostic tests and imaging to ensure accurate diagnosis.
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