Leukocyte Alkaline Phosphatase (LAP)

How is the Test Performed?

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic bandis placed around the upper arm. This causes veins below the band to fill with blood. A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants and young children, the area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.

The white blood cells are separated from the rest of the blood components. Among the enzymes within white blood cells is LAP. It is capable of modifying substances that contain phosphate so they bind to certain dyes. The amount of LAP can be estimated by examining the size and color of the stained granules that form.