CA 19-9 is a tumor marker used in the diagnosis, evaluation of a patient's response to treatment, and monitoring of patients with pancreatic or hepatobiliary cancer. In pancreatic cancer diagnosis, the presence of symptoms such as pancreatic obstruction or biliary obstruction, along with elevated CA 19-9 levels, helps the doctor conclude the risk of pancreatic cancer rather than benign pancreatic tumors. Similarly, patients with symptoms such as ascites, jaundice, and elevated CA 19-9 levels may have an increased risk of liver cancer.