Ask the Doctors

Dear Doctor: How effective is the CA 125 blood test at detecting ovarian cancer? Because ovarian cancer is diagnosed so late, wouldn't it make sense to screen women with this test at the same time they get their Pap smears?

Dear Reader: As you point out, ovarian cancer is often diagnosed when the disease has spread far beyond the ovaries. Only 15 percent of the time is it detected while confined to one of the ovaries; 17 percent of the time, it has spread to local lymph nodes; and 61 percent of the time it has spread distantly. Each year, nearly 14,000 women in the United States die from the disease.

Many people logically think that screening for CA 125 could help reduce those numbers. After all, CA 125 is a glycoprotein that is elevated in 85 percent of ovarian cancers, and blood levels of it are used as a marker for the efficacy of ovarian cancer treatment.

But let's look at the data. In a 1996 study in the United Kingdom, 22,000 women over the age of 45 underwent annual CA 125 tests for three years. Those with a CA 125 level greater than 30 also had an ultrasound of the ovaries. Of the 767 women who had a measurement greater than 30, 49 were ultimately diagnosed with ovarian cancer. The authors determined that the test was able to detect ovarian cancer in only 3.1 percent of cases.

Although an elevated CA 125 was linked to an increased risk of ovarian cancer, the majority of women with elevated numbers did not have ovarian cancer. This high false positive rate has been seen in multiple other studies of CA 125. The fact is, this glycoprotein is elevated in 1 percent of healthy women and can fluctuate during the menstrual cycle. Women with fibroids in the uterus, endometriosis, cirrhosis of the liver and other cancers also may have elevated numbers.

To further evaluate the efficacy of CA 125, in 2011, U.S. researchers conducted a trial involving 78,216 women ages 55 to 74; half received yearly CA 125 tests for six years plus pelvic ultrasounds, and half received usual care. While there was a slight increase in the diagnosis of ovarian cancer in the screening group, there was also a minimal increase in the death rate in that group possibly due to the investigation of false positive tests.

That's not to say some women won't benefit. A 2013 study in the U.K. found that women with a genetic predisposition for ovarian cancer, such as those with BRCA1 or BRCA2 mutations or Lynch syndrome, benefitted from CA 125 screening with the addition of ultrasound. Those who were not vigilant with the screening were more often diagnosed with later-stage ovarian cancer.

The take-home message from these studies: Unless you're at high risk of ovarian cancer or have a family history of the disease, there doesn't appear to be any benefit in a CA 125 test. I wish that were not the case.

Note to readers: A recent column on myasthenia gravis should have said not that symptoms would improve, but that symptoms can stabilize over the course of time and subside in many people. My thanks to those readers who clarified this important point.

(Send your questions to, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

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