The presence of certain cancers can be confirmed by the specific proteins being shed into the blood. These proteins are the tumor markers. Carcinoembryonic antigen (CEA) is the protein linked to colorectal cancers. Every one of us has a little CEA in our blood. The presence of this protein in the blood alone does not mean you have colon cancer. The CEA blood test cannot be used to diagnose cancer, only to track cancer after a diagnosis.