| Subject | Contents |
| Definition | This is a test that measures the amount of the enzyme alkaline phosphatase (LAP) within white blood cells. |
| Alternative Names | |
| How the test is performed | Adult or child: Blood is drawn from a vein ( venipuncture ), usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet (an elastic band) or blood pressure cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to distend (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 tourniquet 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 . Infant or young child: 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 hydrolyzing (liquefying) substrates that contain phosphate into a product that will bind to certain dyes. LAP is roughly quantitated by scoring the size and intensity of color of the stained granules that form. |
| How to prepare for the test | 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. Infants and children: The physical and psychological preparation you can provide for this or any test or procedure depends on your child's age, interests, previous experiences, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child's age:Infant test or procedure preparation (birth to 1 year) Toddler test or procedure preparation (1 to 3 years) Preschooler test or procedure preparation (3 to 6 years) Schoolage test or procedure preparation (6 to 12 years) Adolescent test or procedure preparation (12 to 18 years) |
| How the test will feel | 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. |
| Why the test is performed | Alkaline phosphatase is an enzyme which is present in different forms in many different tissues. LAP (alkaline phosphatase within white blood cells) is useful in confirming the presence of certain disorders, and in distinguishing immature circulating neutrophils. |
| Normal Values | A staining score of 20 to 100 (out of a maximum of 400) is considered normal. |
| What abnormal results mean | LAP is usually increased with: Polycythemia veraMyelofibrosis LAP may be normal or increased with: Leukemoid reaction to infection Women on oral contraceptives LAP is usually decreased with chronic granulocytic leukemia . LAP may be decreased with decreased bone marrow activity: Pernicious anemiaAplastic anemiaPrimary thrombocythemia is an additional condition under which the test may be performed. |
| What the risks are | Excessive bleedingFainting or feeling light-headed Hematoma (blood accumulating under the skin) Infection (a slight risk any time the skin is broken) Multiple punctures to locate veins |
| Special considerations | LAP values are totally independent of serum alkaline phosphatase values. Veins and arteries vary in size from one patient to another, and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others. |
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