The CA 19-9 test is a blood test that measures the levels of a specific protein called CA 19-9. Elevated levels of CA 19-9 in the blood can indicate the presence of certain cancers, particularly pancreatic, liver, and gallbladder cancers. However, it's important to note that CA 19-9 levels can also be elevated in non-cancerous conditions, such as pancreatitis and liver disease. Therefore, the CA 19-9 test is not used as a definitive diagnostic tool for cancer but rather as a tumor marker that can aid in monitoring cancer progression and treatment response. Additional tests and diagnostic procedures are typically required to confirm a cancer diagnosis.