Cancer Antigen 125 (CA-125) is a tumor marker 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. A series of CA-125 tests that shows rising or falling concentrations is often more useful than a single result.

CA-125 is sometimes used along with transvaginal ultrasound to test and monitor women who have a high risk for ovarian cancer but who do not yet have the disease. The most significant risk factor for ovarian cancer is an inherited genetic mutation in one of two genes: breast cancer gene1 (BRCA1) or breast cancer gene 2 (BRCA2). Other risk factors are family history, increasing age, reproductive history and infertility, use of hormone replacement therapy, and obesity.

Sometimes a CA-125 test may be ordered along with a transvaginal ultrasound to help investigate a lump in woman's lower abdominal area (pelvic mass.)

The test is not used, however, to screen women for ovarian cancer because it is non-specific. The U.S. Preventive Services Task Force recommends against screening women for ovarian cancer. This recommendation applies to asymptomatic women but not to women at high risk for developing ovarian cancer, such as those with a genetic mutation.

Currently, there is no one reliable method for early detection of ovarian cancer among asymptomatic women. Less than 20% of ovarian cancers are found in the early stages before they have spread outside the ovary. One reason they go undetected is that the symptoms of ovarian cancer are fairly non-specific. In the meantime, regular physicals, pelvic exams, and an awareness of family history and other risk factors are important. (For more about ovarian cancer, see the "What is being tested?" section or the article on Ovarian Cancer.)

The CA-125 test may be ordered before a woman starts ovarian cancer treatment as a baseline to compare against future measurements. During therapy, a healthcare practitioner may order CA-125 testing at intervals to monitor response to therapy. CA-125 may also be measured periodically after therapy is completed.

In some cases, a healthcare practitioner may order a CA-125 test when a woman has a pelvic mass in order to help determine the cause.

Some healthcare providers may order a CA-125 test and a transvaginal ultrasound at regular intervals when a woman is at high risk of developing ovarian cancer.

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 and is not a good marker of disease progression.

In a woman with a pelvic mass, and in a woman with a high risk for developing ovarian cancer, a significantly elevated CA-125 is a concern but does not necessarily indicate ovarian cancer. This finding would prompt further testing and evaluation.

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.

A new tumor marker, human epididymis protein 4 (HE4), may be ordered along with CA-125 to monitor epithelial cell tumors, the most common type of ovarian cancers. These cancers develop from the cells that cover the outside of the ovaries. The use and full clinical utility of this test has yet to be determined.