Understanding the CA 15-3 Blood Test for Breast Cancer

female scientist, role of lab test CA 15-3 in breast cancer
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Whether you or a loved one are undergoing treatment for breast cancer, it's normal to be overwhelmed with the sheer number of tests ordered by your doctor.

One blood test that may be ordered if you have (or had) breast cancer is called the cancer antigen 15-3 (CA 15-3) biomarker test.

Biomarkers are substances within a person's blood that can be used to monitor the growth of a cancer—they "mark" the presence of the cancer.

Interpreting the CA 15-3 Blood Test in Breast Cancer

CA 15-3 is a protein that is a normal product of your breast tissue. While the CA 15-3 protein does not cause breast cancer, if a cancerous tumor is in your breast, your levels of CA 15-3 may increase as the number of cancer cells increase.

This is because tumor cells may shed copies of the CA 15-3 protein, which can then be measured by a blood test. 

While the CA 15-3 blood test is not used to screen or diagnose breast cancer, there are three other reasons why your doctor may order this blood test: 

Evaluating for Breast Cancer Recurrence 

If you have completed treatment for breast cancer, your doctor may test your blood for CA 15-3 on a regular schedule to see if your levels are rising or remaining steady. Rising levels of CA 15-3 may indicate a recurrence of breast cancer. Since other conditions can cause elevated levels of CA 15-3, however, the test results must be considered along with the results of imaging studies, your symptoms, and other tests.

Monitoring Your Response to Breast Cancer Treatment

Following levels of CA 15-3 may help your doctor know if your treatment is working or not. It's important to note, however, that it may take four to six weeks after treatment before a change is seen in the level of CA 15-3. 

An exception to this, which is important to note, is for those women who are treated with the chemotherapy drug Afinitor (everolimus). Due to the way this medication works against breast cancer, the results of a CA 15-3 biomarker test may actually be the opposite of what is happening with your cancer. In other words, even if your cancer is responding to Affinitor, your CA 15-3 level may rise, and if your cancer continues to progress, your CA 15-3 level may drop, giving the false impression that your disease is improving.

Determining If Your Breast Cancer Has Progressed

If your CA 15-3 level continues to increase, it may suggest that your cancer is worsening. In fact, the level of CA 15-3 is linked with the degree of spread of a breast cancer and is often particularly high when bone metastases and/or liver metastases are present (metastases means that the breast cancer has spread to other organs within the body).

Caveats of the CA 15-3 Blood Test

As mentioned, levels of CA 15-3 tend to be particularly high in women who have liver or bone metastases due to breast cancer, which is why the CA 15-3 tumor marker test is most useful in monitoring the response to metastatic breast cancer treatment.

Even so, there are a few caveats to keep in mind when interpreting your results:

Not Every Breast Tumor Causes a Rise in CA 15-3

For women with breast tumors that do not produce CA 15-3 or with early-stage breast cancers, this test is not useful. In fact, only about 30 percent of women with localized breast cancer (which means the cancer is limited to the breast) will have increased levels of CA 15-3. On the other hand, in women with metastatic breast cancer, CA 15-3 can be found in 50 to 90 percent of all cases.

Other Conditions May Cause an Elevated CA 15-3

If a CA 15-3 blood test is elevated, it doesn't necessarily mean you have metastatic breast cancer or even breast cancer at all. This is because there are several other conditions that may cause an elevated CA 15-3 level like:

CA 15-3 Results Are Not Great at Predicting Prognosis 

While the CA 15-3 blood test is often elevated with the increased spread of metastatic breast cancer, it's not always very good in estimating your prognosis (your chance of recovering from breast cancer). 

In other words, while the CA 15-3 blood test may provide useful information, it's not used to solely guide your breast cancer therapy or monitor how well you are responding to your current therapy.

A Word From Verywell

If your CA 15-3 tumor marker level is elevated, and it's believed to be related to the spread of breast cancer, you and your doctor will look at several other factors in making decisions about further tests as well as treatment.

Some of these factors include the size of your tumor, other tumor marker tests, the results of imaging studies, as well as any findings from your history or physical exam.

The bottom line here is that the CA 15-3 blood test is just one piece of the puzzle when it comes to you or your loved one's breast cancer. There are many factors that go into determining the next step, and your prognosis goes beyond lab values. In particular, the CA 15-3 blood test could lead to anxiety as it may take up to 6 weeks to drop after effective therapy, and with some types of treatment, the level may actually change in a direction that implies the opposite of what is really happening.

Lastly, one of the most important things you can do as a breast cancer survivor is to be your own advocate in your cancer care. In learning about tumor markers such as CA 15-3, you have taken a very important step into doing just that. 

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