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White blood cells in the urine usually indicate a urinary tract infection. The leukocyte esterase (LE) test detects esterase, an enzyme released by white blood cells. Positive test results are clinically significant. The LE test is also used to screen for gonorrhea and for amniotic fluid infections. The combination of the LE test with the urinary nitrite test provides an excellent screen for establishing the presence of a urinary tract infection (UTI). A urine sample that tests positive for both nitrite and leukocyte esterase should be cultured for pathogenic bacteria.
"It has been proposed that the reagent strip for leukocyte esterase designed for the testing of urine (Combur test UX) could be a useful tool for diagnosing [spontaneous bacterial peritonitis] SBP." Braga et al. concluded that the Combur test UX urine screening test is a very sensitive and specific method for diagnosing SBP in the ascitic fluid of cirrhotic patients who have undergone paracentesis."