Cancer Antigen 15-3

The CA 15-3 test is a blood test that doctors use to help them see how well treatment is working for people with breast cancer that has spread to other parts of the body, and sometimes for ovarian cancer too. When doctors test someone’s blood many times over a period, they can see changes in the CA 15-3 levels. These changes usually match up with what’s happening with the cancer. If the level goes down, it often means the treatment is working.