ca19-9 ca19-9 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. it is known to play a role in cell-to-cell recognition processes. it is also a tumor marker used primarily in the management of pancreatic cancer. sialyl-lewisa names systematic iupac name (12s,14s,15r,16r,32r,33r,34s,35s,36r,52r,53s,54r,55r,56ξ,72s,73s,74r,75s,76s)-15,55-diacetamido-14,33,35,56,73,74,75-heptahydroxy-36,52-bis(hydroxymethyl)-76-methyl-16--2,4,6-trioxa-1,7(2),3(4,2),5(4,3)-tetraoxanaheptaphane-12-carboxylic acid other names sialyl lea, slea, cancer antigen 19-9, ca19-9 identifiers cas number 92448-22-1 y 3d model (jsmol) interactive image chebi chebi:61793 chemspider 9160877 mesh sialyl+lewis+a pubchem cid 10985677 unii zej6fm4ujk y smiles o=c(o)1(o2(o)(co)o(o3(o4(o)(o)(o)(c)o4)(co)oc(o)3nc(c)=o)2o)c(o)(nc(c)=o)((o)(o)co)o1 properties chemical formula c31h52n2o23 molar mass 820.748 g·mol−1 except where otherwise noted, data are given for materials in their standard state (at 25 °c , 100 kpa). y verify (what is yn ?) infobox references structure ca19-9 is the sialylated form of lewis antigena. it is a tetrasaccharide with the sequence neu5acα2-3galβ1-3glcnacβ. clinical significance tumor marker guidelines from the american society of clinical oncology discourage the use of ca19-9 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 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. therefore it is useful as a surrogate marker for relapse. in people with pancreatic masses, ca19-9 can be useful in distinguishing between cancer and other diseases of the gland. limitations ca19-9 can be elevated in many types of gastrointestinal cancer, such as colorectal cancer, esophageal cancer and hepatocellular carcinoma. apart from cancer, elevated levels may occur in pancreatitis, cirrhosis, and diseases of the bile ducts. it can also be elevated in people with obstruction of the bile ducts. in patients who lack the lewis antigen a (a blood type antigen on red blood cells), which is about 10% of the caucasian population, ca19-9 is not produced by any cells, even in those with large tumors. this is because of a deficiency of a fucosyltransferase enzyme that is needed to produce lewis antigen a. history ca19-9 was discovered in the serum of patients with colon cancer and pancreatic cancer in 1981. it was characterized shortly after, and it was found to be carried primarily by mucins.