CA 15-3

What it is

CA 15-3 (cancer antigen 15-3) is a protein measured in the blood. It can be raised in some patients with breast cancer, especially in advanced disease, but it also rises in other, non-cancerous conditions.

When it is used

• Monitoring treatment in some patients with metastatic breast cancer: the oncologist compares the value over time, together with symptoms, examination and imaging, to see whether the treatment is working.

The CA 15-3 result should never be the sole basis for changing treatment.

When it is NOT used

CA 15-3 is not a screening test. It should not be done as a “check” for breast cancer in women without symptoms, because:

• many tumors, especially in the early stages, do not raise this marker — a normal value can give false reassurance
• the value can rise in liver disease and other benign conditions — causing anxiety and unnecessary tests

How the result is interpreted

• Raised value ≠ cancer. It must be interpreted by the doctor in context.
• Normal value ≠ no cancer.
• The reference range varies by laboratory and method. For follow-up over time, it is best to use the same laboratory and the same method.
• A small change may simply be biological or laboratory variation.

Preparation

A venous blood sample is taken. Fasting is usually not needed for this test, but may be requested for other tests taken at the same time. Tell the laboratory if you take biotin (vitamin B7, often in supplements for hair and nails), because it can interfere with some measurement methods.

Breast checks — what really matters

If you notice a lump, a skin change, nipple retraction or bloody discharge, seek assessment even if CA 15-3 is normal. Clinical examination and imaging — mammography and/or breast ultrasound — are the right tools for this assessment. Screening is organized according to age and individual risk.