How Does the CA125 Test Work?

As part of the process of checking out endometriosis, your doctor may request some special blood tests called CA125 and CA 19-9. These tests are also known as cancer markers or tumor markers, so having the tests done can be nerve-wracking. What do they really mean?

What is CA125?

CA125 is a protein on the surfaces of certain types of cells in the body. For example, the endometrium (the cells that line the uterus and that are involved in endometriosis), and cells lining other organs like the inside of the abdomen, the heart or lungs, all make CA125. Inflammation causes these cells to make more CA125, and this causes abnormal test results.¹

CA125 levels can often be high in the case of ovarian cancer - for example, if ultrasound shows a cyst or tumor on an ovary. But it’s important to know that many non-cancerous and even normal conditions can cause the test to be abnormal. Something as simple as being on your period can cause abnormal test results. Fibroids, pregnancy, normal ovarian cysts, or taking hormone replacement therapy can all cause abnormal results.² Endometriosis also often, but not always, causes an abnormal CA125 test.³

What is CA19-9?

CA19-9 is a test that, like CA125, can be abnormal in cases of both ovarian cancer and endometriosis. According to a meta-analysis, the result is more likely to be abnormal in cases of more advanced endometriosis (stage III and IV).³ But, like CA125, an abnormal test may not mean that the person has either cancer or endometriosis; Gallstones, for example, can cause an abnormal CA19-9 test.⁴

What does it all mean?

So, it’s complicated. If ultrasound shows a growth on an ovary, for instance, your doctors may ask you to have these tests done. The results can help them  get a better idea if it is an endometrioma (chocolate cyst) or a cancer. It’s not a yes-or-no result, but added to other information can help make the best decisions about what to do.

Bottom line: An abnormal CA125 or CA 19-9 test DOES NOT mean you have cancer, especially if you already know that you have endometriosis. An abnormal test doesn’t necessarily mean that you have endometriosis either. Endometriosis is just one of many non-cancerous conditions that can cause an abnormal test. But endometriosis doesn’t always cause an abnormal test.

Talk to your doctor about what the results may mean in your particular situation, so that you can make a plan together.