A CEA (carcinoembryonic antigen) test is a blood test that measures the level of CEA protein in the blood. CEA is a protein that is normally found in the tissue of a developing baby in the womb, and the blood level of this protein disappears or becomes very low after birth. In adults, an abnormal level of CEA may be a sign of cancer. However, an increased CEA level alone cannot diagnose a new cancer, and further testing is needed. The CEA test is most commonly used in people who have been diagnosed with cancers that often increase CEA levels, such as colorectal cancer. The test can help predict the likelihood of recovery and/or the chance that cancer will come back after treatment, check how well cancer treatment is working, and see if cancer has returned after treatment. The normal range for CEA is 0 to 2.5 ng/mL (0 to 2.5 µg/L), and in smokers, slightly higher values may be considered normal (0 to 5 ng/mL, or 0 to 5 µg/L) . However, an increased CEA value can occur in both cancers and non-cancerous conditions, and further tests, procedures, follow-up care, or additional treatment may be needed if the result is abnormal.