Leukocyte esterase (LE) is an enzyme produced by white blood cells, specifically neutrophils, which are the body’s primary responders to bacterial invasion. Measuring LE in a urine sample is a quick, non-invasive method used as part of a complete urinalysis. A positive result signals that the body is mounting an immune response somewhere in the urinary tract.
What Leukocyte Esterase Is
Leukocyte esterase is released exclusively by activated white blood cells, particularly neutrophils. When infection or inflammation is present in the urinary tract, neutrophils migrate to the site and release LE into the surrounding fluid. The presence of LE in the urine serves as a chemical indicator that white blood cells are present in the sample. This condition, characterized by a significant number of white blood cells, is medically known as pyuria. A negative LE test is the expected result, as normal urine contains an insignificant number of these cells.
Interpreting the Test Result
The test for leukocyte esterase is most commonly performed using a chemical reagent strip, or dipstick, briefly immersed in the urine sample. Chemicals on the strip react with the LE enzyme, causing a color change if it is present. Results are reported qualitatively, ranging from negative to trace, and then positively graded as 1+, 2+, or 3+. A positive LE result strongly suggests pyuria, indicating inflammation or an ongoing immune response within the urinary system.
This test is frequently interpreted alongside the result for nitrites, another chemical marker found on the dipstick. Nitrites are produced when certain types of bacteria, such as E. coli, convert urinary nitrates. When both LE and nitrites are positive, the probability of a bacterial urinary tract infection is significantly increased. A positive LE result with a negative nitrite result can still indicate an infection caused by non-nitrite-producing bacteria, or it may point to an inflammatory condition that is not bacterial in origin.
Causes of a Positive Result and Next Steps
The most frequent cause for a positive leukocyte esterase test is a bacterial infection anywhere along the urinary tract. This includes common conditions such as cystitis (bladder inflammation) or urethritis (urethra inflammation). A positive result can also indicate a more serious kidney infection, known as pyelonephritis. Non-infectious causes, such as kidney stones, certain forms of kidney disease, or contamination during sample collection, can also trigger a positive result.
Because the LE test is a screening tool and not a definitive diagnosis, a positive result requires further investigation to determine the cause. The next step is typically a microscopic analysis of the urine, where a technician counts the number of white blood cells and looks for bacteria. Physicians will also order a urine culture, which is the gold standard for identifying the specific type of bacteria present. The culture helps the healthcare provider select the most appropriate antibiotic treatment.