A carcinoembryonic antigen is a glycoprotein, or protein containing carbohydrates, which is produced in the large intestines of a developing fetus. Its production normally stops before birth, but the antigen sometimes is found in the blood of a healthy adult in very low levels. If the antigen is found in an adult in higher levels, it is considered a tumor marker, indicating some form of cancer. Types of cancer may include cancer in the colon, rectum, pancreas, breast, thyroid, ovaries or lungs. Certain non-cancer conditions may also trigger these antigen levels to increase. Conditions may include cholecystitis, or swelling of the gallbladder, pancreatitis, peptic ulcer and inflammatory bowel diseases, among many others.
Measurement of carcinoembryonic antigen levels is generally done through a blood test. No special preparations are needed for the test, except for people who smoke. Carcinoembryonic antigen levels tend to increase with smoking, and patients are often instructed to avoid smoking for a short period before blood is taken. After application of a tourniquet or rubber tie on the patient’s upper arm and cleaning the puncture site with an antiseptic, blood is drawn from a vein in the arm, and then processed in the laboratory where the carcinoembryonic antigen level is determined.
For most individuals, carcinoembryonic antigen levels in the blood are normally between zero and 2.5 micrograms per liter (mcg/L). People who smoke have higher levels, with values between zero to 5 mcg/L. Higher results may indicate cancer. The test, however, is not specific to cancer alone, as antigen levels may also increase due to other medical conditions, such as cirrhosis or diverticulitis.
The carcinoembryonic antigen blood test is also helpful in determining an individual's response to cancer treatments. Measurement of the antigen before and after medical and surgical interventions is important in the evaluation of treatment success as well as the assessment of a patient's chances for a full recovery. For instance, before a patient undergoes surgery to remove a cancerous tumor, his antigen levels are determined, providing a baseline record of his level. This level is often compared to measurements taken after surgery to see if it has fluctuated. Levels also are commonly measured before and after a chemotherapy session. Surgery and chemotherapy are often considered successful when serum carcinoembryonic antigen returns to normal levels. Persistent elevations of carcinoembryonic antigen may help doctors predict a recurrence of cancer.