CA19-9 (Carbohydrate Antigen 19-9) is a carbohydrate antigen, belonging to the category of oligosaccharide tumor-associated antigens, and is a glycolipid on the cell membrane. It is formed from the precursor substances of Lewis antigens (including Le-a and Le-b) under the joint action of sialyltransferase and fucosyltransferase. CA19-9 is an important tumor marker, especially in the diagnosis and monitoring of pancreatic cancer, where the levels of CA19-9 are elevated in over 80% of patients with pancreatic cancer. CA19-9 can also be used in the clinical auxiliary detection of gastric cancer, colorectal cancer, gallbladder cancer, cholangiocarcinoma, and liver cancer.