Carcinoembryonic Antigen Test (CEA). A carcinoembryonic antigen (CEA) test is a blood test used to help diagnose and manage certain types of cancers, especially cancer of the colon.The test measures the amount of CEA present in the blood. If you already have cancer, this test helps a doctor determine if the treatment for the cancer is working.An antigen is a harmful substance that is released by cancerous tumors. CEA test is most useful to monitor patients who already have been diagnosed with a type of cancer that is known to produce CEA. Not all cancers produce CEA. Increased levels of CEA may be found in the following cancers: 1  colorectal (colon) cancer. 2  medullary thyroid carcinoma.