CA 19-9, also known as Cancer Antigen 19-9 and Carbohydrate Antigen 19-9, is a sialylated derivative of Lewis A blood group antigen. CA 19-9 is currently recognized as one of the most common tumor markers for colorectal, gastric, and hepatocellular cancers. Increases in levels of biomarkers CA 19-9, CA242, and CEA inversely correlate with survival time in pancreatic adenocarcinoma patients. Serum levels of CA 19-9 are elevated in 70 - 80% of pancreatic cancer patients and may also be elevated in a variety of other cancers and benign conditions like biliary tract obstruction. Elevated levels are associated with advanced disease at presentation and disease progression during follow-up. High CA 19-9 levels correlate with unresectable or more advanced tumors, although this evaluation has not been widely used to establish inoperability. High marker levels may also be used to predict patient outcomes and are used to monitor treatment response and recurrence.