ALP isoenzyme test

Definition

Alkaline phosphatase (ALP) is a substance found in all body tissues. There are many different forms of ALP. Each type has a different chemical structure, called an isoenzyme. Its structure depends on where in the body it is produced. For example, liver and bone ALP isoenzymes have different structures.

The ALP isoenzyme test is a laboratory test that measures the amounts of different types of ALP in the blood.

See also: Alkaline phosphatase test

Alternative Names

Alkaline phosphatase isoenzyme test

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 site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.

Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

How to prepare for the test

You should not to eat or drink anything for 10 to 12 hours before the test, unless otherwise instructed by your doctor.

Many drugs affect the level of alkaline phosphatase in the blood. 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.

• Allopurinol
• Antibiotics
• Anti-inflammatory medicines
• Birth control pills
• Certain arthritis drugs
• Certain diabetes medicines
• Chlorpromazine
• Cortisone
• Male hormones
• Methyldopa
• Narcotic pain medicines
• Propranolol
• Tranquilizers
• Tricyclic antidepressants

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 may be used to diagnose:

• Liver disease
• Biliary disease (jaundice)
• Parathyroid disease
• Vitamin D deficiency
• Upper abdominal pain
• Bone disease

It may also be done to check liver function and to see how medicines you take may affect your liver.

Normal Values

The normal value is 20 to 140 IU/L (international units per liter). Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

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.

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 tumors
• Osteomalacia
• Rickets
• Skeletal disease
• Anemia
• Rickets
• Leukemia
• Thyroid gland inflammation
• 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

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:

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)

Special considerations

This test is about 80% accurate for identifying the specific locations of cancers or disease. However, it is not a reliable screening test because levels may be high for unknown reasons and return to normal. Unless there are signs of a disease, slightly higher-than-normal values of ALP in the CHEM-20 test are not considered significant.

References

Berk PD, Korenblat KM. Approach to the patient with jaundice or abnormal liver test results. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 150.

Drezner MK. Osteomalacia and rickets. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 265.