Urinalysis involves assessment of the physical properties of urine, such as color and clarity; chemical analysis using a urine test strip; and microscopic examination. Microscopic examination is not always included: it may be reserved for samples that have abnormal results on preliminary examinations, or performed at the healthcare provider's request. Test strips contain pads impregnated with chemical compounds that change color when they interact with specific elements in the urine; for example, nitrite, a compound produced by some bacteria that cause urinary tract infections, and leukocyte esterase, an enzyme found in white blood cells (WBCs) which acts as an indicator of the number of WBCs in the urine. The intensity of the color change correlates roughly with the concentration of each compound.