CEACAM5 and gastric cancer: Tumor markers are absent or present in small quantity in healthy tissues.[8] A number of blood derived tumor markers are routinely tested before surgery for GC patients, such as carcinoembryonic antigen (CEA), carbohydrate antigen 19-9 (CA19-9), alpha fetoprotein (AFP), carbohydrate antigen 72-4 (CA72-4), and carbohydrate antigen 15-3 (CA 15-3).