Carcinoembryonic Antigen Test (CEA) is a type of blood test which is used to diagnose various kinds of cancer. This test is basically used to rectify cancers present in intestine and rectum. It is also used after the cancer is diagnosed and helps to determine if the cancer treatment is working or not. An Antigen is a substance that is formed by cancerous cells and is sometimes present in the bloodstream. The CEA test indicates the amount of CEA in your blood. If a very high amount of CEA is still present in the blood, then the test indicates that cancer is gone or has spread to other parts.