Leukocyte alkaline phosphatase (LAP)

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

How the test is 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.

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.

For infants and children:
The physical and psychological preparation you can provide for this procedure depends on your child’s age, previous experiences, and level of trust. For specific information regarding how you can prepare your child, see the following topics:

  • 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 present in different forms thoughout the body. LAP (alkaline phosphatase within white blood cells) is useful in confirming the presence of certain disorders.

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 higher-than-normal with:

  • Polycythemia vera  
  • Myelofibrosis

LAP may be normal or higher-than-normal with:

  • Leukemoid reaction to infection  
  • Women on oral contraceptives

LAP is usually lower-than-normal with chronic granulocytic leukemia.

LAP may be lower-than-normal with decreased bone marrow activity:

  • Pernicious anemia  
  • Aplastic anemia

Primary thrombocythemia is an additional condition under which the test may be performed.

What the risks are  

  • Excessive bleeding  
  • Fainting 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  
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.

Johns Hopkins patient information

Last revised: December 4, 2012
by Janet G. Derge, M.D.

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