What is the CA-125 Marker?

The CA-125 marker is a protein that is often found at higher levels in some patients with cancer. A CA-125 test is a blood test that may be used to evaluate an elevated risk for ovarian cancer or other reproductive cancer in some women, or that can be part of the ways physicians assess how well ovarian cancer is responding to treatment. In this test, blood is tested for increased levels of the CA-125 marker, indicating that cancer may be present.

The CA-125 marker test is often misunderstood as a method of cancer screening that could be applicable to all women, but this is inaccurate. It is not used as a routine test for all women because there are so many ways the CA-125 test can generate a reading that suggests cancer — either from other physical conditions or benign sources. This doesn't mean the test isn't useful, but that its uses are extremely specific.

A woman receiving a CA-125 test undergoes a very simple blood draw which doesn't require fasting beforehand. After the test, women are usually free to resume normal activities, though they may need to wear an adhesive bandage at the blood draw site for a few hours. Blood collected is analyzed for cancer antigen 125 (CA-125), which is determined by counting the units per milliliter (U/mL). A reading above 35 U/mL might suggests presence of cancer in the ovaries, in the fallopian tubes, or in the lining of the uterus because the antigen is generally present in higher amounts when these cancers are present.

It has to be stressed that a reading above 35 U/mL may not be suggestive of anything. When a woman is pregnant or menstruating, her CA-125 test results could be this high or higher. Women who have conditions like endometriosis or pelvic inflammatory disorder also frequently have high CA-125 marker readings. Such results are not a diagnosis of cancer, and only if a doctor suspected cancer would he or she consider other tests that might confirm or rule out these suspicions.

There are a few instances where it is necessary to get a CA-125 test. During treatment for ovarian or other reproductive cancers, it can make sense to get an initial reading and then continue to evaluate cancer antigen 125. If treatment is successful, it's likely that antigen levels will decline over time.

Another positive way the CA-125 test can be used is as a diagnostic tool, along with others, to look for rising levels of the cancer marker in women who have genes that make them more predisposed to develop ovarian cancer. Should antigen levels increase, doctors would want to perform other diagnostic tests to see if cancer had developed. Even in these cases, an increase in CA-125 marker levels wouldn't necessarily indicate cancer.