Alkaline phosphatase (ALP) is a substance found in all body tissues. There are many different forms of ALP. Each type is different and is called an isoenzyme. Its structure depends on where in the body it is produced, such as the liver and bones.
The ALP isoenzyme test is a lab test that measures the amounts of different types of ALP in the blood.
A blood sample is needed. For information on how this is done, see: Venipuncture.
How to prepare for the test
You should not eat or drink anything for 10 to 12 hours before the test, unless your doctor tells you to do so.
Your health care provider may tell you to stop taking certain drugs before the test. Never stop taking any medicine without first talking to your doctor. The following drugs may affect the level of alkaline phosphatase in the blood:
Birth control pills
Certain diabetes medicines
Narcotic pain medicines
Nonsteroidal anti-inflammatory drugs (NSAIDs, used for arthritis and pain)
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
When the alkaline phosphatase (ALP) test result is high, the doctor may order the ALP isoenzyme test. This test will help determine what part of the body is causing higher ALP levels.
It may also be done to check liver function and to see how medicines you take may affect your liver.
The normal value is 20 to 140 IU/L (international units per liter).
Adults have lower levels of ALP than children. Bones that are still growing produce higher levels of ALP. During some growth spurts, levels can be as high as 500 IU/L. For this reason, the test is usually not done in children, and abnormal results refer to adults.
The isoenzyme test results can reveal whether the increase is in "bone" ALP or "liver" ALP.
Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.
The examples above show the common measurements for results for these tests. Some laboratories use different measurements or may test different specimens.
Levels that are only slightly higher than normal may not be a problem unless there are other signs of a disease or medical problem.
What the risks are
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.
Other risks may include:
Fainting or feeling light-headed
Hematoma (blood accumulating under the skin)
Infection (a slight risk any time the skin is broken)
Pratt DS. Liver chemistry and function tests. In: Feldman M, Friedman LS, Brandt LJ, eds. Sleisenger and Fordtran's Gastrointestinal and Liver Disease. 9th ed. Philadelphia, Pa: Saunders Elsevier;2010:chap 73.
David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.