Granulocytic white blood cells (ie, neutrophils, basophils and eosinophils) contain esterases. The detection of esterase is the basis of the leukocytes test on the urine reagent strip. Leukocyte esterase reacts with the reagents on the reagent strip (indoxy ester derivative and diazonium salt) to produce a purple color. The intensity of the color produced is proportional to the amount of leukocyte esterase that is present.
The granulocytes that are most often detected in urine are neutrophils. Their presence indicates a probable urinary tract infection.