Blood tumor markers are substances that are secreted by the tumor cells themselves or stimulated by the body to produce landmarks when tumor diseases occur in the body. In other words, when the human body suffers from certain types of tumors, there will be abnormally high concentrations of certain substances, which are called tumor markers. There are various clinical tumor markers, the most common ones are alpha-fetoprotein, which is the most common indicator of liver tumor, and the concentration of alpha-fetoprotein is very low in normal people. When liver cancer is under control, the concentration of fetoprotein will drop significantly, but when liver cancer recurs again, the concentration of fetoprotein will increase significantly. Similarly, the concentration of carcinoembryonic antigen, which is an indicator of human digestive system tumors, such as pancreatic cancer, colon cancer and gastric cancer, may also increase abnormally.