CA 15-3

CA 15-3 is a blood test that measures a protein fragment from MUC1, a mucin glycoprotein that can be shed into the bloodstream by some breast cancer cells. Many healthy cells make MUC1 on their surface, but certain cancers release more of it, which can be detected by sensitive immunoassays.

Clinicians use CA 15-3 mainly to help monitor known breast cancer over time, especially in advanced or metastatic disease. It is not a screening test for people without cancer and it is not used by itself to diagnose breast cancer. Not all breast cancers produce CA 15-3, so some people with active disease may have low or unchanged levels.

For people with metastatic or recurrent breast cancer, CA 15-3 can help track response to treatment and watch for changes that might suggest progression. Trends in this marker, when interpreted alongside your symptoms, physical exam, and imaging, can provide an earlier signal of how therapy is working or whether a change in care should be considered.

CA 15-3 is not specific to breast cancer. It can be increased by other cancers and by some noncancer conditions, so it should not be used alone to make treatment decisions. Your care team considers the whole clinical picture. The test itself requires only a standard blood draw, which carries minimal risks such as brief discomfort or bruising.

A single CA 15-3 result rarely tells the whole story. Your team looks for patterns over time, ideally using the same laboratory method for consistency. Falling results during therapy can be reassuring, while rising results may prompt closer follow-up or imaging. Sometimes there is a temporary rise after starting an effective treatment, so context and timing matter.

If a result does not match how you feel or what imaging shows, your clinician may repeat the test, check for assay interferences, or use additional tests. Do not start or stop treatments based only on this marker. Discuss your results with your care team so you can plan next steps together.