Alkaline Phosphatase (ALP) Test
A blood sample taken from a vein in the arm
Fasting is preferred but not required for this test
Alkaline phosphatase is an enzyme found in high levels in bone and liver. Smaller amounts of ALP are found in the placenta of women who are pregnant, and in the intestines. Each of these body parts makes different forms of ALP. The different forms are called isoenzymes.
How is the sample collected for testing?
A blood sample is taken by needle from a vein in the arm.
Is any test preparation needed to ensure the quality of the sample?
Fasting is preferred but not required for this test. Eating a meal can increase the ALP level slightly for a few hours in some people. It is usually better to do the test after fasting overnight. In this case, only water is permitted.
How is it used?
When a person has evidence of liver disease, very high ALP levels can tell the doctor that the person’s bile ducts are somehow partially or totally blocked or inflamed. Often, ALP is high in people who have cancer that has spread to the liver or the bones, and doctors can do further testing to see if this has happened. If a person with bone or liver cancer responds to treatment, ALP levels will decrease. When a person has high levels of ALP, and the doctor is unsure why, ALP isoenzymes might be requested to try to determine the cause.
When is it requested?
What does the test result mean?
Raised levels of ALP are usually due to a disorder of either the bone or liver. If other liver function tests such as bilirubin, gamma-glutamyl transferase (GGT) or alanine aminotransferase (ALT) are also raised, this usually indicates that the ALP is coming from the liver. However if calcium and phosphate measurements are abnormal, this suggests that the ALP might be coming from bone. In some forms of liver disease, such as hepatitis, ALP is usually much less elevated than AST or ALT. However, when the bile ducts are blocked (for example by gallstones, scars from previous gallstones or surgery, or by a tumour), ALP and bilirubin may be increased much more than either AST or ALT. ALP can also be raised in bone diseases such as Paget’s disease (where bones become enlarged and deformed), in certain cancers that spread to bone or in vitamin D deficiency.
Is there anything else I should know?
Pregnancy can increase ALP levels. Children have higher ALP levels because their bones are growing, and ALP is often very high during the 'growth spurt' which occurs at different ages in males and females. Temporary elevations of ALP are also seen with healing fractures. Transient benign increases in ALP may also be seen in young infants.
Eating a meal can increase the ALP level slightly for a few hours in some people. Ideally the test should be done after fasting overnight. Some drugs may increase ALP levels, especially some of the drugs used to treat psychiatric problems or epilepsy but significant increases are rare.
What other tests are used to evaluate liver disorders?
Who is at risk for a liver disorder?
While many things could harm the liver, the major causes of liver disease are infection by viruses and the drinking of too much alcohol. In rare cases, some medicines can damage the liver; your doctor may suggest liver tests from time to time if you are taking any of these drugs. Some diseases inherited from your parents can also occasionally damage the liver.
What signs should make me aware of a liver disorder?
In many cases, liver disease has few symptoms. When the liver is damaged rapidly (a disease called acute hepatitis), the skin and the whites of the eyes often turn yellow, the urine turns brown, and bowel movements lose their colour. In its late stages, liver disease may cause confusion, easy bruising, swelling in your abdomen, and the vomiting of blood.