What is a CA-125 Test?

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  • Written By: wiseGEEK Writer
  • Edited By: O. Wallace
  • Last Modified Date: 28 December 2018
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CA-125 stands for cancer antigen 125, and this is a protein which possibly shows up in a CA-125 test when women have ovarian cancer. Measures of this protein are calculated by taking a blood test. If the test yields positive results, this might be indication to have further exams to see if ovarian cancer is in fact present. The trouble with the test is that it is known for significant inaccuracy and can readily yield both false positive and false negative results.

There are uses for the CA-125 test. When ovarian cancer is present and being treated, measurements of CA-125 may help track treatment and possibly determine cancer stage. This is a legitimate use of the test, though other testing is often done too to assess cancer stage or determine recovery.


Interest in using the CA-125 test as a cancer predictor surged with the wide circulation of a viral email in the late 1990s and 2000s, that was written by an actual person, Carolyn Benevigna. The experience she writes of would surely inspire most people to want the test, since she was tested for ovarian cancer through numerous means and only discovered its presence by taking the test. She begged people to insist that they ask their doctors to perform this test, and implied that doctors might not do so because of costs. She has since revised her statement, but the email still gets circulated, even though her experience is by no means an accurate account of how testing in early stages might help all people.

In fact, the American Cancer Society and several other organizations suggest that using the CA-125 test to detect early cancer is incredibly unreliable. Numerous things can cause false positive testing including fibroids, endometriosis, pregnancy, diseases of the liver, or infections. False negative testing is also fairly common, and the test may fail to detect early stages of cancer. It becomes more reliable as cancer progresses, but isn’t likely to be accurate until later stages of cancer are reached. When the test is accurate, these results are called true negatives or true positives.

What is important for women to understand about the CA-125 test is that it does not generally say whether a woman has or doesn’t have ovarian cancer. It may be helpful in later stages of cancer to determine treatment, but the test is not a diagnosis of cancer and it does not rule out the possibility of ovarian cancer. More reliable methods of looking for cancer include ultrasound to visualize ovaries, and performing vaginal and rectal physical examinations.



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