What does the Cancer Antigen 15-3 test measure?

Why is Cancer Antigen 15-3 measured?

CA 15-3 is used to monitor disease activity and treatment response in patients with established metastatic breast cancer. It is not recommended for screening, diagnosis, or staging of early breast cancer because sensitivity and specificity are inadequate in those settings. Serial measurement can detect biochemical progression or response months before imaging changes, supporting decisions to continue or alter systemic therapy, but results must always be integrated with clinical and radiological findings rather than dictating management in isolation.

Reference ranges

Standard reference

Typically <30–31 U/mL; assay-dependent.

Functional medicine optimal

Not applicable; CA 15-3 is a tumour marker interpreted by trend and magnitude of change in the context of known malignancy, not by an 'optimal' concentration in healthy populations.

Units

U/mL, kU/L

Reference ranges vary by laboratory and assay platform. Interpret in context of the issuing laboratory's stated range.