Carcinoembryonic Antigen (CEA) is a blood test that measures the level of carcinoembryonic antigen in the blood. CEA is a protein that is normally present at high levels in fetal blood but decreases to low levels in healthy adults. Elevated levels of CEA may indicate certain types of cancer, including colorectal, lung, breast, thyroid, liver, and stomach cancers. The CEA test is used to monitor the progression of cancer, evaluate treatment effectiveness, and detect cancer recurrence.