The CA125 antigen and what it means in ovarian cancer

19 Aug 2021

If you are diagnosed with ovarian cancer, you might be asked for a CA125 blood test at some point. To help understand why that is, we asked Associate Professor Sam Saidi, a gynae-oncologist at the Chris O’Brien Lifehouse, to explain it a little more.

To start with, could you please tell us what CA125 is and why it needs to be measured in some cases?
“So the CA125 is what we call a tumour marker and that is a serum marker in the blood that under normal circumstances is at a certain level and in some conditions might be detected at a higher level. Those conditions can be anything including ovarian cancer, some other cancers, as well as benign conditions, such as endometriosis. Even, for instance, an episode of food poisoning or any other infection or any disturbance of your liver function might make your CA125 rise.”

Can the CA125 test be used to detect ovarian cancer?
“Well, there is a common misconception, even with doctors, that these tests will pick up ovarian cancer before it becomes problematic, yet all the studies that have looked into it have not been able to show that. One reason is that we don't know how ovarian cancer develops, whereas, with cervical cancer, breast cancer, and bowel cancer (the ones we do a national screening for), we know how those cancers arise and how long they take to arise. And they have specific warning signs in specific tests that we can look at before they become cancer. Ovarian cancer doesn't do that. When studies have been done specifically, the CA125 fails as a warning sign, so no, it doesn't work. That's why ovarian cancer is not subjected to a screening program, as the other cancers are.”

So, you wouldn’t be asked for a CA125 test just because you are at high risk of ovarian cancer?
“No, you wouldn't normally have it measured just for being a high risk of ovarian cancer. The indication for measuring it is if you go to the doctor with some symptoms that might be suspicious of ovarian cancer, then they would measure it. Or if you've had treatment for ovarian cancer, we would measure it because it can be a useful marker of your response to treatment.”

How effective then is a CA125 test for women who have been diagnosed with ovarian cancer?
“That's a good question. Usually, the CA125 test after treatment (once you have ovarian cancer) is a good indicator of response to treatment. So, if you have ovarian cancer with a high CA125, and then you get treatment and the CA125 drops, we are very reassured that you are responding very well to treatment. And the reason for that is because the CA125 is only high because you've got cancer, in most cases. And so, once we have treated cancer, and we see a drop in the CA125, then we know the cancer is responding to the treatment. It's not a direct correlation, but it is a correlation.”

And what about in cases of reoccurrence?
“Yes, so once you've had treatment, what usually happens is your CA125 remains at a baseline level. And then at recurrence, especially if you had a high CA125 when you first got your cancer, then when cancer returns, your CA125 often rises first before you get symptoms. This can help guide further treatment”