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Alkaline phosphatase (ALP)

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Alternative Names: ALK PHOS, ALKP, serum alkaline phosphatase (SAP, SALP)


Alkaline phosphatase (ALP) is an enzyme that can be found in many body tissues including the liver, bone, intestines and placenta. Depending on the location of the enzyme, ALP is divided into several forms called isoenzymes (also called isozymes). Alkaline phosphatase found in the blood is the total amount of all isoenzymes from all tissues. In adults ALP is mainly represented by liver and bone isoenzymes in approximately equal amounts. Other isoenzymes are present in small amounts.

The alkaline phosphatase test is usually performed to diagnose conditions associated with the hepatobiliary system (the liver, bile ducts and the gallbladder) or bone disease. Diseases that destroy the cells of organs containing alkaline phosphatase lead to the release of ALP into the blood, which raises the blood level of alkaline phosphatase.

Elevated levels of ALP in the blood occurs with all forms of cholestasis (a condition where bile cannot flow from the liver to the duodenum), especially with obstructive jaundice.

Alkaline phosphatase in bone is produced by special cells called "osteoblasts" that are responsible for forming new bone. Any condition that affects the growth of bones or causes increased activity of bone cells can affect the level of ALP in the blood. The ALP test can be helpful in identifying, for example, cancer that has spread to the bone or it can also be used to diagnose Paget's disease (a disease in which one or more bones are affected by a pathological bone reconstruction). The ALP test can also sometimes be used to monitor the treatment of Paget's disease or other bone conditions, such as vitamin D deficiency.

Normal levels of ALP

A small amount of ALP is present in the blood because the enzymes are released during normal cell turnover. However, in case of trauma or pathologic processes the level of ALP increases significantly.

The normal range of ALP can vary from laboratory to laboratory depending on the age, gender, the method used for the test.

Elevated Levels of ALP

Abnormally high ALP in the blood is most often associated with liver disease or bone disorders. For example, ALP is especially high when the bile ducts are blocked. A less significant increase of ALP levels can be seen with use of drugs toxic to the liver, in liver cancer and cirrhosis, and in hepatitis. ALP may be increased in any condition that causes excessive bone formation, including Paget's disease, as well as other diseases such as rheumatoid arthritis and healing fractures.

If other liver tests such as bilirubin, ALT (alanine aminotransfere), AST (aspartate aminotransferase) and GGT (gamma-glutamyl transferase) are also elevated, usually the increase in ALP is associated with the liver. But if GGT level is normal, then ALP may be increased due to bone condition. Increased levels of calcium and phosphorus in the blood usually indicate that the ALP is coming from bone.

The ALP levels can be increased during pregnancy due to the placental isoenzymes. Children and adolescents normally have higher ALP levels than adults because their bones are growing, and ALP is often very high during a growth spurt, which occurs at different ages in boys and girls.

Low ALP levels

Low ALP levels are less common than elevated levels. Lowered levels of ALP in the blood may be caused by a deficiency of zinc and magnesium in the body, a rare genetic bone disorder called hypophosphatasia, malnutrition, blood transfusions.


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