Carcinoembryonic Antigen (CEA) is a protein not normally detectable in the blood of a healthy person. When the protein appears in the blood of an adult, it can indicate cancer, but it will not indicate which kind of cancer is present. CEA is often used to monitor patients with cancers of the gastrointestinal (GI) tract typically after surgery to measure the response to therapy and whether the disease is recurring. It can also indicate benign conditions.

A blood test for CEA is often used as a tumor marker. Physicians can use CEA results to determine the stage and extent of disease and the outlook in patients with cancer, especially gastrointestinal (GI) and, in particular, colorectal cancer. CEA is also used as a marker for other forms of cancer. It has been found helpful in monitoring patients with cancer of the rectum, lung, breast, liver, pancreas, stomach, and ovary. Not all cancers produce CEA; therefore, the CEA test is not used for screening the general population.