Tumor marker tests look for tumor markers, sometimes called cancer markers, in the blood, stool, urine, or body tissues. Tumor markers are substances made by cancer cells or by normal cells in the body in response to cancer. Because tumor markers can also occur in some noncancerous conditions, tumor marker tests are generally not used to diagnose cancer or to screen people at low risk for the disease. These tests are most often done on people who have already been diagnosed with cancer.Q-Check Tumor marker tests can help determine whether your cancer has spread, if your treatment is working, or if your cancer has returned after treatment.