A laboratory test that measures the level of CA 19-9 in the blood. CA 19-9 is a substance released into the blood by both cancer cells and normal cells. Higher than normal amounts of CA 19-9 in the blood can be a sign of pancreatic or other types of cancer or other conditions. The amount of CA 19-9 in the blood can be used to help keep track of how well cancer treatments are working or if cancer has come back. CA 19-9 is a type of tumor marker.