CA 19-9 is a tumor marker that is used primarily in the management of pancreatic cancer. CA 19-9 is an antigen defined by monoclonal antibody binding to CA 19-9, the tumor surface marker Sialyl-Lewis A. Guidelines from the American Society of Clinical Oncology discourage the use of CA 19-9 as a screening test for cancer, particularly pancreatic cancer. The reason is that the test may be falsely normal (false negative) in many cases, or abnormally elevated in people who have no cancer at all (false positive). The main use of CA 19-9 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 people with pancreatic masses, CA 19-9 can be useful in distinguishing between cancer and other diseases of the gland.