CA 15.3 (Breast Cancer Marker)

CA 15.3 (cancer antigen 15.3) is a protein found in the blood that can be raised in certain conditions affecting the breast. It is best known as a tumour marker for breast cancer, where levels can be elevated, and it is used mainly to help monitor the condition rather than to detect it in the first place.

What it can help assess

• Monitoring known breast cancer, by tracking how levels change during and after treatment to help assess response and check for recurrence
• Following advanced or metastatic breast cancer, where it is most useful, as levels tend to reflect the amount of active disease
• Supporting the wider clinical picture, alongside examination, imaging, and other tests

An important note on interpretation

CA 15.3 is not a screening or diagnostic test for breast cancer. Levels are often normal in early breast cancer, so a normal result does not rule it out, and it should never be used to detect the disease in someone without a diagnosis. Levels can also be raised by a number of non-cancerous conditions, including benign breast and liver conditions. For these reasons the result is always interpreted alongside your full clinical picture rather than on its own, and it is most valuable in people already diagnosed with breast cancer.

Why it matters

For people diagnosed with breast cancer, CA 15.3 is a useful tool for monitoring the condition over time and assessing how well treatment is working. Any result should be discussed with your GP or specialist, who can interpret it in the context of your individual situation and advise on the appropriate next steps.