Leukocyte esterase is tested in a urine sample when a medical professional suspects that the patient has a urinary tract infection, according to WebMD. Leukocytes are white bloods cells, which are not normally present in urine except in cases of urinary infection.
The leukocyte esterase test is just one component of a comprehensive urinalysis if a urinary tract infection is suspected, says WebMD. The odor and turbidity of the urine can also indicate an infection, as well as the presence of nitrites or even bacteria or parasites in the sample. The presence of leukocytes in a urine sample does not guarantee a urinary tract infection, so additional testing may be needed.