Alkaline phosphatase (ALP) is an enzyme (protein) produced in slightly different forms (isoenzymes) by several body tissues, including the bones, liver, bile ducts, intestine and placenta. Elevated levels of alkaline phosphatase in the blood can therefore be due to different causes (or isoenzymes), most often liver or bone damage. With some exceptions, the analysis of ALP isoenzymes is not useful in pregnant women (due to increased placental isoenzymes) or when total ALP levels are normal.
A high percentage of liver and/or biliary ALP may be observed when the channels through which bile circulates are blocked by inflammation (cholecystitis ) or a bile duct stone (liver stone), in cases of hepatitis, cirrhosis or liver cancer. A high level of GGT, another biliary enzyme, confirms these possibilities. A high percentage of bone ALP can be caused by any condition that increases bone formation: growth (childhood), Paget’s disease, osteomalacia, recent fracture, etc. An increase in the percentage of intestinal ALP is most often of no clinical significance and can occur in some individuals (often of blood type O) after a high-fat meal. These increases are sometimes family-related and the percentages return to normal after fasting.