CARCINOEMBRYONIC ANTIGEN - A glycoprotein that is often detected in the serum of patients with colon cancer. The carcinoembryonic antigen (CEA) can be assayed for in the blood. This can be a useful test to monitor a colon cancer patient's response to chemotherapy. CEA levels may be elevated in a variety of conditions including breast cancer, colon cancer, lung cancer, pancreatic cancer, thyroid cancer, bladder cancer, ulcerative colitis, diverticulitis, cholecystitis, liver disease, peptic ulcer disease, emphysema, and lung infections. CEA levels may be higher in those who smoke heavily.