In a woman with known ovarian cancer, a rise in her CA-125 usually means a progression or recurrence of the disease. A decrease in the CA-125 usually means the disease is responding to treatment.
In a woman who has NOT already been diagnosed with ovarian cancer, an elevated CA-125 can mean a number of things. While it can indicate that she has ovarian cancer, it can also indicate other types of cancer, as well as several benign diseases such as endometriosis.
To understand why the CA-125 is not a good general screening test, it is important to understand some basic facts about diagnostic tests. If the result of a diagnostic test is abnormal, the test result is usually considered to be "positive" (meaning the person seems to have the disease). If the result is normal, the test is considered "negative" (the person doesn't seem to have the disease). However, tests are somewhat imperfect, and the results might be incorrect.
Therefore, we are left with four possibilities every time a test is done:
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True positive -- means that the test result is abnormal and the patient does, in fact, have the disease.
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False positive -- means that even though the test result was abnormal, the patient in fact does NOT have the disease.
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True negative -- means the test is normal and the patient does not have the disease.
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False negative -- means that even though the test is normal, the patient does in fact have the disease, and that the test simply missed it.
The less "false positive" and "false negative" results any test gives, the more accurate it is. Some tests might be very accurate in certain groups of people, but not accurate in other groups.
This is the situation for the CA-125 test. In women with a known diagnosis of ovarian cancer, an elevation of the CA-125 is almost always an indicator of a cancer recurrence. In other words, a positive test usually means the disease is present. (The "false positive" rate is low.)
However, when used in a group of healthy women, an elevated CA-125 usually does NOT mean ovarian cancer is present. The vast majority of healthy women with an elevated CA-125 do not have ovarian cancer (or any other cancer for that matter). The "false positive" rate for this group of women is high.
In fact, only about 3 out of 100 healthy women with elevated CA-125 actually have ovarian cancer. Any woman with an abnormal CA-125 test will need further tests, and sometimes invasive surgical procedures, to confirm the result. These additional tests all involve risks and anxiety. On the rare occasion when cancer is found, it is usually not even at the earliest stage.
Therefore, the CA-125 should not be considered an effective general screening test for ovarian cancer. Studies are underway to determine whether it might be effective when combined with other blood tests or radiologic studies.
