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CA19-9 (carbohydrate antigen 19-9 or sialylated Lewis (a) antigen) is a blood test from the tumor marker category. While it is not useful to diagnose particular forms of cancer, specifically pancreatic cancer, it may be useful to monitor the results of treatment and to determine whether the disease may be recurring. Other causes for elevated CA19-9 levels include Mirizzi's syndrome and other hepatobiliary diseases. In patients that have the Lewis antigen blood type (about 5% of the population), CA19-9 is not elevated in pancreatic cancer even with large tumors because they have a deficiency of fucosyltransferase that is needed to produce CA19-9.
Ca19-9 was discovered in patients with colon cancer and pancreatic cancer in 1981.