The CEA test is a vital 'tumor marker' test used by specialists. It measures a protein that is normally low in healthy adults. This test is not a screening tool for the general population. Its essential, gold-standard use is for monitoring patients who have been diagnosed with certain cancers, particularly colorectal cancer. It helps your doctor see how you are responding to treatment and provides a key, early-warning signal for any recurrence. A simple blood draw provides this invaluable data for your long-term care.