A positive result on a WBC, or leukocyte, esterase test is considered abnormal, according to MedlinePlus. This raises the possibility of a urinary tract infection. False positives are sometimes caused by vaginal secretions or an infection caused by the Trichomonas vaginalis parasite.
A health professional places a dipstick in a urine sample, and a color change indicates a potential problem, states MedlinePlus. Negative results are normal. However, high levels of protein or vitamin C sometimes lead to false negatives. The WBC esterase test specifically looks for signs of infection, such as white blood cells, in urine. If a dipstick color change occurs, the urine sample undergoes microscopic examination to double-check results.