CEA, or Carcinoembryonic Antigen, is a protein that can be found in the blood. It is often used as a tumor marker, particularly in diagnosing and monitoring certain types of cancer, such as colorectal cancer. Elevated levels of CEA may indicate the presence of cancer, but they can also be influenced by other conditions, including smoking and inflammatory diseases. Doctors typically measure CEA levels through a blood test. While it is not used for screening cancer in the general population, it can help assess treatment effectiveness and detect recurrence in patients already diagnosed with cancer. Regular monitoring of CEA levels can provide valuable information for managing patient care.