The WBC esterase can be a sign of a urinary tract infection. It is an enzyme that is released by white blood cells. Your urinalysis did show that you had some epithelial cells, so it was not a great specimen (not a good clean catch) and was contaminated with some vaginal skin cells.he WBC esterase can be a sign of a urinary tract infection. It is an enzyme that is released by white blood cells. Your urinalysis did show that you had some epithelial cells, so it was not a great specimen (not a good clean catch) and was contaminated with some vaginal skin cells.