Tumor markers are substances produced by cancer cells or other cells in the body in response to cancer. These substances can be found in the blood, urine, or tissue of people with cancer. Tumor marker tests are used to: Help detect cancer: Some tumor markers are elevated in the early stages of cancer, making them useful for screening. Help diagnose cancer: If other tests suggest cancer, a tumor marker test can help confirm the diagnosis. Monitor cancer treatment: Tumor marker levels can be measured during and after treatment to see if the cancer is responding. Check for cancer recurrence: Rising tumor marker levels after treatment may indicate that the cancer has returned. Common Tumor Marker Tests: There are many different tumor markers, each associated with specific types of cancer. Some common examples include: Alpha-fetoprotein (AFP): Liver cancer, testicular cancer, ovarian cancer Carcinoembryonic antigen (CEA): Colorectal cancer, lung cancer, breast cancer Prostate-specific antigen (PSA): Prostate cancer Cancer antigen 125 (CA 125): Ovarian cancer Cancer antigen 19-9 (CA 19-9): Pancreatic cancer, colorectal cancer Human chorionic gonadotropin (HCG): Testicular cancer, ovarian cancer, gestational trophoblastic disease