Synonyms:  CA-125 (Cancer Antigen) Test, Carbohydrate Antigen (CA-125) Test Why It Is Done • Check if treatment for cancer is working. If the level of CA-125 is going down, it usually means that the treatment is working. • Check if cancer has returned. • Check if the ovary is the main site of cancer in a woman. Test Overview Cancer antigen 125 (CA-125) measure the amount of this protein in the blood. CA-125 is found on the surface of many ovarian cancer cells. It also can be found in other cancers and in small amounts in normal tissue. CA-125 is used as a tumor marker—which means some types of cancer are present. Most often, the CA-125 test is used to check how well treatment for ovarian cancer is working or to see if ovarian cancer has returned How To Prepare No special preparation required