CA-125 is a protein that is more prevalent in ovarian cancer cells than in other cells. This protein enters the blood stream and can be measured by a blood test. There are two CA-125 tests, a first and second generation test. The second generation test is now more widely used and is generally more accurate.

The test is most appropriately used to follow women who have already been diagnosed with ovarian cancer. In these cases, the CA-125 is a very good indicator of whether a patient is responding to treatment for her cancer, and whether a patient remains in remission after treatment. In general, the CA-125 is not a good test to screen healthy women for ovarian cancer.

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.

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