Carbohydrate antigen 19-9 (CA19-9), also known as sialyl-LewisA, is a tetrasaccharide which is usually attached to O-glycans on the surface of cells.

In people with pancreatic masses, CA19-9 can be useful in distinguishing between cancer and other diseases of the gland.

[1] Apart from cancer, elevated levels may occur in pancreatitis, cirrhosis,[1] and diseases of the bile ducts.

[3] In people who lack Lewis antigenA (a blood type antigen on red blood cells), which is about 10% of the white population, CA19-9 is not produced by any cells,[3] even in those with large tumors.

[2] This is because of a deficiency of a fucosyltransferase enzyme that is needed to produce Lewis antigenA.