CA-125 is used to monitor therapy during treatment for ovarian cancer. CA-125 is also used to detect whether cancer has come back after treatment is complete. This test is sometimes used to follow high-risk women who have a family history of ovarian cancer but who do not yet have the disease.

Before a patient starts treatment for ovarian cancer, the physician may order a baseline CA-125 to compare against future measurements. During therapy, physicians use CA-125 testing at intervals to monitor response to therapy. CA-125 may also be measured periodically after therapy is completed. An increase in CA-125 may indicate that the cancer has returned.

If CA-125 levels fall during therapy, this generally indicates that the cancer is responding to treatment. If CA-125 levels rise, the cancer may not be responding to therapy. High CA-125 levels after treatment is complete may indicate that the cancer has come back.

Because CA-125 can be high in many normal or benign conditions (for example, pregnancy, menstruation, endometriosis, pelvic inflammatory disease), it is not useful as a screening test in large populations.