CA 15-3 (breast cancer marker)
A protein shed by some breast cancer cells that can be measured in the blood. It is not a screening test for breast cancer; its main use is to help monitor women already diagnosed with breast cancer — tracking whether treatment is working or the disease is changing over time.
What it measures
CA 15-3 is a protein made by the same gene family that produces mucins on the surface of cells, and it can be shed into the blood by some breast cancer cells. Its blood level is used almost entirely as a monitoring tool in people already known to have breast cancer, rather than to find cancer in the first place. In someone being treated for or followed after breast cancer, a CA 15-3 level that is falling suggests treatment is working, while a rising trend over repeated tests can be an early clue that the disease is becoming more active, prompting closer assessment with imaging. It is the trend across several measurements, not a single number, that carries the information. Importantly, CA 15-3 is not specific to cancer: mild elevations occur in a range of benign conditions, including non-cancerous breast and liver disease, and it is often normal in early breast cancer — which is exactly why it is not used for screening. It is always interpreted by the specialist team alongside examination and imaging, as one thread in a larger picture rather than a stand-alone answer.
What a high value can mean
- Known breast cancer becoming more active — the main concern when the level rises over serial tests.
- Advanced or metastatic breast cancer — often higher and used to follow response.
- Benign breast or liver conditions — can cause mild, non-cancer elevations.
- Other cancers — occasionally raised in ovarian, lung or other tumours, so it is not specific.
- A single mildly raised value — frequently has no serious cause and needs the trend and clinical context.
What a low value can mean
- Normal or low — the expected result, and reassuring during monitoring.
- A falling level during treatment — suggests the therapy is working.
- Not a rule-out — a normal CA 15-3 does not exclude breast cancer, especially early disease.
When to discuss with a doctor
CA 15-3 is a specialist monitoring marker, not a screening or diagnostic test — a raised or normal value on its own tells you little without the clinical context. In someone treated for breast cancer, it is followed over time: a rising trend across serial tests is a signal to look more closely with imaging, while a falling or stable level is reassuring. Outside that setting, a mildly raised CA 15-3 is common from benign conditions and rarely means cancer. Because early breast cancer often does not raise it, it is never used to screen or to rule cancer out. Mediora.AI can surface a rising CA 15-3 trend and show it alongside related markers, but this needs specialist assessment — the interpretation belongs with your oncology team.