CA 15-3 (Cancer Antigen 15-3)

What this test is

CA 15-3 is a blood test linked to breast cancer. In someone already being treated for breast cancer that has spread beyond the breast, doctors sometimes use it, along with scans, to see how well treatment is working. That is a specialist decision. For most other situations, it does not help. It is not a screening test to find breast cancer early, and it is not used to watch for cancer coming back after successful treatment, because studies have not shown it catches problems sooner than a checkup would, or that it helps people live longer. It can also read a little high for reasons that have nothing to do with cancer, which can cause worry and extra testing without benefit. If you have a breast lump or other concern, an exam and imaging are the right first steps.