Leukocyte alkaline phosphatase (LAP)

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This is a test that measures the amount of an enzyme called alkaline phosphatase within white blood cells (leukocytes).

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 band is 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. LAP causes substances that contain phosphate to bind to certain dyes. The amount of LAP can be estimated by examining the size and color of the stained granules that form.

Fast for 6 hours before the test. The health care provider may advise you to discontinue drugs that can affect the test, including some antibiotics, narcotics, methyldopa, propranolol, cortisone, allopurinol, tricyclic antidepressants, chlorpromazine, oral contraceptives, anti-inflammatory medications, androgens, tranquilizers, some antiarthritic drugs, and oral (by mouth) anti-diabetic drugs.

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Alkaline phosphatase is an enzyme present in different forms throughout the body. LAP (alkaline phosphatase within white blood cells) is useful in confirming the presence of certain disorders.

Review Date: 02/03/2005
Reviewed By: Rita Nanda, M.D., Department of Hematology/Oncology, University of Chicago Medical Center, Chicago, IL. Review provided by VeriMed Healthcare Network.