This test measures a protein called carcinoembryonic antigen (CEA) in your blood. This protein is found on some types of cancer cells. CEA is called a tumor marker because it gives information about cancer in your body. This test may help your healthcare provider find out if your cancer treatment is working. It may also help your provider know if the cancer is coming back after treatment. The test can be used with cancers of the colon and rectum, lung, breast, pancreas, ovary, prostate, liver, and thyroid.