CA-125 is primarily 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. Series of CA-125 tests that show rising or falling concentrations are often more useful than a single result. This test is sometimes used to test and monitor high-risk women who have a family history of ovarian cancer but who do not yet have the disease.

This test is not used to screen for ovarian cancer because it is non-specific. Levels in the blood can be elevated in other conditions such as normal menstruation, pregnancy, endometriosis, and pelvic inflammatory disease.

A doctor may order a CA-125 test before a woman starts ovarian cancer treatment as a baseline to compare against future measurements. During therapy, physicians order 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 or stay the same, then the cancer may not be responding to therapy. High CA-125 levels after treatment is complete may indicate that the cancer has come back.

If a woman who has been diagnosed with ovarian cancer has a baseline CA-125 level that is normal, then the test is not likely to be useful to monitor her ovarian cancer. In this case, the ovarian cancer may not be producing CA-125 so it is not a good marker of disease progression.

Because CA-125 can be high in many normal or benign conditions, such as pregnancy, menstruation, endometriosis, and pelvic inflammatory disease, it is not useful or recommended as a screening test for the general population.