A positive leukocyte screening test determines the possible existence of a urinary tract infection, according to MedlinePlus. Positive screens mean medical professionals should use microscopes to examine patients' urine for white blood cells and other signs of infection. Heavy mucus discharge, blood, other vaginal secretions and trichomonas infections, such as trichomoniasis, can cause false positive results. A negative test result for leukocyte esterase testing is normal. High levels of vitamin C or protein can produce false negative test results.
Generally, white blood cells contain the enzyme leukocyte esterase, notes the American Association for Clinical Chemistry. It is normal for a few white blood cells to be present in urine. A high white blood cell count in urine can indicate an inflamed kidney or urinary tract, which produce positive results in leukocyte esterase testing. Bladder, kidney or other bacterial urinary tract infections are the typical causes for increases in the numbers of white blood cells in urine.
Laboratories testing patients' urine for leukocyte esterase use test strips and color changes on test pads to estimate the amount of protein in the urine, explains the American Association for Clinical Chemistry. When a small amount of protein is present in the urine, the test pad produces a minor color change. A large amount of protein in the urine produces a significant color change and positive screen test results.