Granulocytic white blood cells in a urine sample suggest the presence of a urinary tract infection. Granulocytes, which include neutrophils, basophils and eosinophils, contain esterases. These esterases catalyze the strip reagent indoxylcarbonic acid ester to release indoxyl. Indoxyl reacts with a diazonium salt to produce a purple color. The intensity of the color produced is proportional to the amount of enzyme present.