ALP

Alkaline phosphatase (ALP) is an enzyme found in several tissues throughout the body, including liver, bone, kidney, bowel (intestine), and in the placenta of women who are pregnant. However, the highest concentrations ALP are present in the cells that comprise the bone and liver. In the liver, it is found on the edges of cells that join to form bile ducts - tiny tubes that drain bile from the liver to the bowels where it is needed to help digest fat in the diet. ALP in bone is produced by special cells called "osteoblasts" that are involved in the formation of bone. Each of the various tissue types produces distinct forms of ALP called isoenzymes.

Elevated levels of ALP in the blood are most commonly caused by liver disease or bone disorders. Levels of the enzyme can be greatly increased, for example, in cases where one or more bile ducts are blocked. Smaller increases of blood levels are seen in liver cancer and cirrhosis, with use of drugs toxic to the liver, and in hepatitis. Any condition causing excessive bone formation, including bone disorders such as Paget's disease and others such as rheumatoid arthritis and healing fractures, can cause increased ALP levels. Children and adolescents typically have higher blood ALP levels because their bones are still growing.

It is possible to distinguish between the different forms (isoenzymes) of ALP produced by different types of tissues in the body. If it is not apparent from clinical signs and symptoms whether the source of a high ALP test result is from liver or bone disease, then a test may be performed to determine which isoenzyme is increased in the blood. This can help establish whether the increase is due to bone or liver disease.

A blood sample is taken by needle from a vein in the arm.

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.