The CA 19-9 test is a blood test used to monitor pancreatic and gastrointestinal cancers. CA 19-9 is a type of protein produced by cancer cells. This test is often used to monitor the progression of cancer after treatment or to detect early recurrence. High levels of CA 19-9 may be a sign of pancreatic cancer, colorectal cancer, stomach cancer, or other gastrointestinal diseases. However, this test is not used to diagnose cancer because CA 19-9 levels can also be elevated in some other benign conditions.