Alp Isoenzyme Test

This blood test measures the amounts of different forms of the enzyme alkaline phosphatase, or ALP, in the blood. These different forms of ALP are called isoenzymes.

Who is a candidate for the test?

Diseased or damaged tissues release ALP into the blood, causing the ALP level to be higher than normal. However, ALP occurs in all tissues, so a basic ALP test doesn’t tell the healthcare provider what tissue is damaged. Different tissues release different isoenzymes of ALP that can be identified in the laboratory. Blood tested for ALP isoenzymes can therefore help reveal the part of the body that is damaged. Isoenzymes are usually only ordered if the total or basic ALP level is increased and the healthcare provider is not sure why.

How is the test performed?

To measure levels of ALP isoenzymes, a blood sample is taken from a vein in the arm. First, the skin over the vein is cleansed with an antiseptic. A tourniquet is wrapped around the upper arm to enlarge the veins. A small needle is gently inserted into a vein, and blood is collected for testing in the laboratory. After the tourniquet is removed a cotton ball will be held over the needle site until bleeding stops.

What is involved in preparation for the test?

A person should request specific instructions from his or her healthcare provider. Generally, no preparation is required.

What do the test results mean?

Normal values for ALP isoenzymes vary based on the age and sex of the person and the type of test used.

There are 4 main ALP isoenzymes: bone, liver, intestine and placenta. The placenta isoenzyme is usually only seen in pregnant women.

Determining which isoenzyme is elevated tells the healthcare provider which organ may be damaged. For example, if the bone isoenzyme is elevated, bone disease is suspected.