The Carcinoembryonic Antigen (CEA) test is a blood test that measures the level of CEA protein in the blood. CEA is a substance produced by certain types of cancer cells, particularly in colorectal, pancreatic, lung, and breast cancer. The CEA test is used as a tumor marker to detect and monitor cancer progression and treatment effectiveness. However, it is important to note that a high level of CEA does not necessarily indicate the presence of cancer, as it can also be elevated in non-cancerous conditions such as smoking, liver disease, and infections. Additionally, CEA levels may be low or normal in some cancer patients. The CEA test may be used in conjunction with other diagnostic tools, such as imaging tests and biopsies, to aid in cancer diagnosis and treatment planning.