Leukocyte esterase is a specific leukocyte (white blood cell) protein. The leukocyte esterase test is used to quickly determine whether there is a significant number of leukocytes in the urine. The leukocyte esterase test detects intact or broken leukocytes. Therefore, it might not be possible to confirm the test under a microscope when overly diluted urine (density less than 1.006) causes the leukocytes to burst. Leukocytes are abundant in vaginal secretions, and specimen contamination is frequent.
A positive urine leukocyte esterase test indicates either a low urinary tract (bladder and urethra) infection or specimen contamination. The chances of it being a urinary tract infection over a specimen contamination are greatly increased if the person presents the appropriate symptoms and their nitrite test is also positive. A positive leukocyte esterase test can also be seen in the absence of bacterial infection in the case of chronic interstitial nephritis, as caused by a kidney stone (lithiasis), a tumour of the bladder or urethra, a severe bladder infection (cystitis), other inflammation processes or in the presence of atypical infections (chlamydia, ureaplasma, tuberculosis).