A positive (+) result of leukocyte esterase in urine is indicative of the presence of WBC (White Blood Cells), also called leukocytes, in the urine.
The leukocyte esterase is an enzyme secreted by granulated white blood cells (WBC) and monocytes. Granulated white blood cells are those that have granules in their cytoplasm, such as neutrophils, basophils, and eosinophils. When there is a urinary tract infection (UTI) there is an increase in the WBCs in the urine. Neutrophils are the WBCs most frequently associated with bacterial infections. Lymphocytes are the only WBC cells that do not contain esterases and they are not usually the type of WBC present in an infection.
The leukocyte esterase test is an indirect way to study the possible presence of WBCs in the urine where it is not necessary to visualize WBCs under microscope. This test is a standardized means for the detection of WBCs.
The test has also an advantage over the microscopic examination because the test detects both broken (lysed) and intact WBC cells. The disadvantage is that the test is not designed to measure the concentration of WBCs. The quantitation must be done by microscopic examination.
Esterases also are present in histiocytes and Trichomonas (a protozoan parasite that causes trichomoniasis).
The presence of leukocyte esterase in the urine (leukocyte esterase positive +) usually suggests a urinary tract infection (UTI) or trichomoniasis (an infectious disease caused by the parasite Trichomonas vaginalis).
This test is usually performed at the same time of the nitrites in urine test. A positive leukocyte esterase test result is most frequently accompanied by the presence of bacteria, which produce a positive nitrite reaction.
In women with dysuria (painful urination), a positive test for nitrites and leukocyte esterase are usually an indication of a urinary tract infection (probability is higher than 90%). On the other hand, if leukocyte esterase is positive but the nitrite test is negative suggests a vaginal infection (trichomoniasis or chlamydia) or a chronic interstitial nephritis.
For pregnant women, the leukocyte esterase test may be used to test for infection in the amniotic fluid.
This test is performed along with other urine tests. The patient must collect a sample of the urine in a specific container using a special kit. This sample will be sent to the lab for the analysis. The urine should be collected by a clean-catch midstream to avoid bacterial contamination. It is important to avoid also contamination by vaginal secretions.
Leukocyte esterase in urine can be tested either with a dipstick or with a tablet that change of color in the presence of a high amount of esterases in the urine. The dipstick is read at 2 minutes by comparing with a color chart. Color changes that occur after 2 minutes have no diagnostic value.
In laboratories, there are automated machines that perform the analysis, but it is also possible to buy strips that let you analyze a urine sample at home. They comprise up to 10 or more different reagents or chemical pads to perform different urine tests at the same time.
The leukocyte esterase test must be negative to be considered within normality.
A positive result of leukocyte esterase (leukocyte esterase +) in the urine is helpful for diagnosis.
The most common causes for a positive result in this test are:
Urine with a positive result from the dipstick should be examined microscopically and followed up with a urine culture.
Some drugs or conditions may interfere with the leukocyte esterase test: