WHAT IS CA 19-9 (CARBOHYDRATE ANTIGEN 19-9, GICA, GASTROINTESTINAL CANCER ANTIGEN) TEST?
The MUC1 gene is a high-molecular-weight glycoprotein found in specific tissues throughout the body (see Mucin-like carcinoma-associated antigen—Blood). The MUC1 gene has many varieties of carbohydrate chains that are termed mucin-like antigens, and one of these is the carbohydrate antigen 19-9 (CA 19-9) that circulates in the bloodstream when cancer is present. CA 19-9 is a glycoprotein present on a wide variety of adenocarcinomas of the gastrointestinal and hepatobiliary systems. CA 19-9 is produced in excess by the adenocarcinomas and released into the blood, enabling measurement. CA 19-9 is considered the standard of comparison often used a marker for pancreatic cancer, along with CT, to differentiate from a benign pancreatic disease. A total of 70%-80% of pancreatic cancers, 60% of hepatobiliary cancers, and 50%-60% of gastric carcinomas have elevated CA 19-9 levels. In pancreatic adenocarcinoma, 96% of tumours with CA 19-9 levels >1000 U/mL are considered unresectable. Serial elevated postoperative CA 19-9 levels often predict relapse of pancreatic carcinoma before clinical or radiographic findings, but the CA 19-9 levels are not often monitored because of the paucity of effective treatment for relapsed pancreatic carcinoma. The CA 19-9 glycoprotein is not expressed in Lewis (a- b-) individuals (nonsecretors), who account for approximately 7% of the U.S. population and approximately 20% of the population of Japan, leading to the possibility of false-negative results when CA 19-9 levels are being obtained. Because of the lack of high sensitivity and specificity of CA 19-9, it has not previously been considered useful as a screening test for early pancreatic cancer. However, it is currently being studied in conjunction with endoscopic ultrasound as a possible method for early detection. Of note, CA 19-9 is measured using a double monoclonal immunoassay.