Although a urinary tract infection is the most common cause of leukocyte esterase in urinalysis some other and rarer causes for the positive result with a negative microorganism culture do exist. [2] These include:1  Chlamydia. 2  Infection with a sexually transmitted microorganism called ureaplasma urealyticum.ometimes a test for leukocyte esterase produces negative results but later follow-up indicates a disease condition like a urinary tract infection. This could happen from a number of causes a simple as the laboratory technician not allowing enough time before reading the dipstick.