Alkaline phosphatase (ALP or AKP) is an enzyme widely distributed in human tissues such as liver, bones, intestine, kidney and placenta to be excreted outside the bile via the liver. This enzyme catalyzes the removal of the 5′ phosphate group from a nucleic acid molecule, thereby converting the 5′-P end of a DNA or RNA fragment to a 5′-OH end. However, it is not a single enzyme, but a group of isoenzymes. Six isoenzymes, AKP1, AKP2, AKP3, AKP4, AKP5 and AKP6, have been identified. The first, second and sixth are from the liver, the third from bone cells, the fourth from the placenta and cancer cells, and the fifth from the small intestinal villous epithelium and fibroblasts. Clinically, the measurement of ALP is mainly used for the diagnosis and differential diagnosis of diseases of the skeletal, hepatobiliary system, especially for the differential diagnosis of jaundice. For unexplained high ALP serum levels, isoenzymes can be measured to help clarify their organ origin. (1) Physiological increase In children, alkaline phosphatase activity can be 1 to 2 times higher than normal during the physiological period of skeletal development. It can be increased in adolescents during the growth phase, in pregnant women and after eating foods with high fat content. (2) Pathological elevation ① Bone diseases such as rickets, chondromalacia, bone malignancy, malignant tumor bone metastasis, etc.; ② Hepatobiliary diseases such as extrahepatic bile duct obstruction, hepatocellular carcinoma, cirrhosis, capillary hepatitis, etc.; ③ Other diseases such as hyperparathyroidism. (3) Pathological decrease is seen in severe chronic nephritis, childhood thyroid insufficiency, anemia, etc.