ALP (alkaline phosphatase) isoenzyme

A blood test to measure the amounts of different forms of alkaline phosphatase.

Alkaline phosphatase is an enzyme in the blood, intestines, liver, and bone cells. Its chemical structure varies (called isoenzymes) depending on where it is produced. This makes it possible to determine where a problem has originated. When bones are growing, liver cells are damaged, or a biliary obstruction occurs, alkaline phosphatase levels rise considerably.

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 band is placed around the upper arm to apply pressure and restrict blood flow through the vein. 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.

For an 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.

How to prepare for the test
Fasting for 10 to 12 hours before the test is usually standard. A urine specimen may also be tested (your health care provider determines which tests are appropriate). Many drugs affect the level of alkaline phosphatase in the blood. These drugs will be monitored or you will be instructed to discontinue their use:

  • antibiotics  
  • narcotics  
  • methyldopa  
  • propranolol  
  • cortisone  
  • allopurinol  
  • tricyclic antidepressants  
  • chlorpromazine  
  • estrogens and progestins - oral contraceptives  
  • anti-inflammatory analgesics - oral  
  • male hormones  
  • tranquilizers  
  • some antiarthritic drugs  
  • antidiabetics - oral

For infants and children:
The preparation you can provide for this test depends on your child’s age and previous experience. For specific information regarding how you can prepare your child, see the following topics:

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
This test is useful in diagnosing:

  • liver disease (jaundice-yellowing of the skin and eyes)  
  • the cause of liver disease  
  • parathyroid disease  
  • Vitamin D deficiency  
  • the cause of pain in the upper abdomen  
  • bone diseases

It can also be used to monitor:

  • patients on medications that may be harmful to the liver  
  • liver production (used in conjunction with other tests)

Normal Values

The normal value is 20 to 140 IU/L (international units per liter)

Adults have lower levels of ALP than children because children’s bones are still growing. During some growth spurts, levels can be as high as 500 IU/L. Usually children are not measured because of the potential for such high amounts, so the abnormal results refer to adults.

The isoenzymes can reveal whether the increase is in “bone” ALP or “liver” ALP.

What abnormal results mean

Higher-than-normal ALP levels may indicate:

  • pregnancy  
  • healing bone fracture  
  • liver diseases  
  • biliary obstruction  
  • hepatitis  
  • bone disease  
  • Paget’s disease  
  • osteoblastic bone cancers  
  • osteomalacia  
  • rickets  
  • skeletal disease  
  • anemia  
  • rickets  
  • Leukemia  
  • thyroid gland infection  
  • hyperparathyroidism  
  • chronic alcohol ingestion

Lower-than-normal levels of ALP may indicate:

  • protein deficiency  
  • magnesium deficiency  
  • too much vitamin D or too little vitamin C  
  • poor nutrition

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

This test is about 80% accurate for identifying specific locations of cancers or disease. It should not be relied on for a screening test because sometimes levels are high for unknown reasons and return to normal. Unless there is evidence of a disease, higher-than-normal values of alkaline phosphatase in the Chem-20 test are not considered significant.

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 3, 2012
by Levon Ter-Markosyan, D.M.D.

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