Carcinoembryonic antigen (CEA) is a protein that is a tumor marker. In Adults, CEA levels are generally very low in the blood but can be elevated in a wide variety of different types of cancers. CEA levels can also be elevated in non-cancer related conditions, such as cirrhosis, peptic ulcer, ulcerative colitis, rectal polyps, and also in smokers. CEA testing is also done to monitor the cancer during treatment.