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