1 Inflammation of the urinary tract and ureters or infection in bladder could also be the cause of leukocytes esterase in urine. 2  One more reason for leukocyte esterase in urine, is sexual intercourse. 3  During sexual intercourse the bacteria may get pressed into the urethra.idneys, bladder, ureters and urethra form the urinary system and when analysis of urine shows leukocytes (without nitrates) in urine it indicates infection in the system. To diagnose leukocytes, urinalysis has to be done in order to know the level of leukocytes esterase in urine.