CA 19-9 (carbohydrate antigen 19-9, also called cancer antigen 19-9 or sialylated Lewis (a) antigen) 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.
CA 19-9 can be elevated in many types of gastrointestinal cancer, such as colorectal cancer, esophageal cancer and hepatocellular carcinoma. Apart from cancer, elevated levels may also occur in pancreatitis, cirrhosis, and diseases of the bile ducts. It can be elevated in people with obstruction of the bile ducts.
In patients who lack the Lewis antigen (a blood type antigen on red blood cells), which is about 10% of the Caucasian population, CA 19-9 is not expressed, even in those with large tumors. This is because of a deficiency of a fucosyltransferase enzyme that is needed to produce CA 19-9 as well as the Lewis antigen.
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