CA19-9 is a blood test from the tumor marker category. It was discovered in patients with colon cancer and pancreatic cancer in 1981. Increased levels of CA19-9 are also found in non-malignant conditions, such as Mirizzi's syndrome and diseases of the bile ducts and liver. Guidelines from the American Society of Clinical Oncology discourage the use of CA19-9 as a test for cancer, particularly pancreatic cancer. The main use of CA19-9 is therefore to see whether a pancreatic tumor is secreting it; if that is the case, then the levels should fall when the tumor is treated, and they may rise again if the disease recurs.In patients who lack the Lewis antigen, which is about 5% of the population, CA19-9 is not elevated in pancreatic cancer even with large tumors because they have a deficiency of a fucosyltransferase enzyme that is needed to produce CA19-9 as well as the Lewis antigen.