CA 15-3 test: what it shows and how the result is read

CA 15-3 (cancer antigen 15-3) is a mucin-type protein that some breast cancer cells produce in raised amounts in the blood. According to MedlinePlus, tumour markers in general are substances made by cancer cells, or by the body in response to cancer, and their amount is measured in blood, urine or biopsy samples. CA 15-3 is most often used not to make a diagnosis but to follow the treatment of breast cancer that is already known and advanced — whether the treatment being given is working, and whether the disease is not coming back after treatment has ended. Tumour marker tests are of limited use as a screening tool: not every cancer has a known marker, and benign conditions unrelated to cancer can also raise the amount of a marker. That is why a raised CA 15-3 alone is not sufficient grounds for a diagnosis. The result is always assessed together with imaging tests, biopsy findings and the overall clinical picture, and decisions about the next steps are made by the treating oncologist.

• To follow treatment in people who already have a diagnosis of advanced breast cancer — whether the treatment being given is working
• After treatment for breast cancer — to watch whether the disease has come back
• When a doctor wants an additional measure alongside imaging tests to assess how the disease is going
• Together with CA 27-29 — another breast cancer marker used for the same purpose

No single, universally agreed numeric reference limit for the CA 15-3 test is given in public sources, because the result depends on the method the laboratory uses. You will find the exact reference interval on your own laboratory report, and the result is one to discuss with your treating doctor.

A raised CA 15-3

In advanced breast cancer, a raised CA 15-3 can indicate that the disease is progressing or that the treatment is not working well enough. However, according to MedlinePlus, benign conditions unrelated to cancer can also raise the amount of a tumour marker, so a raised value on its own proves nothing — the result is assessed together with imaging tests and the clinical picture.

• Progression of breast cancer, or an insufficient response to the treatment being given
• Benign conditions (unrelated to cancer) that can temporarily raise the amount of a tumour marker
• Other cancers in which this marker can also rise

• Discussing the result with the treating oncologist alongside the findings of imaging tests
• Assessing whether there is a condition unrelated to cancer that could have affected the result
• Repeating the test over time — the direction of change is often more important than a single result
• CA 27-29 or other measures, if the doctor considers it appropriate

A falling CA 15-3

A falling CA 15-3 value during treatment is usually taken as a favourable sign that the treatment being given is working. According to MedlinePlus, this marker is used precisely to follow how well treatment is working in advanced breast cancer, so the direction of change over time matters more than a single separate result.

• A favourable response to the breast cancer treatment being given
• The cause of an earlier rise (for example a temporary benign condition) having resolved

How a falling result is followed up

• Assessing the result together with imaging tests and the overall clinical condition
• Continuing monitoring on the schedule set by the doctor

Related pages: AFP, PSA, how to prepare for a blood test and how to read blood test results.