The CA 15-3 test is a blood test that measures Cancer Antigen 15-3, a tumor marker mainly used in the monitoring of breast cancer. Elevated CA 15-3 levels may indicate disease progression, recurrence, or response to treatment in patients already diagnosed with breast cancer. It is not recommended as a screening test, as levels can be normal in early cancer and may also rise in non-cancerous conditions such as liver disease or benign breast disorders. The test is usually interpreted along with clinical findings and imaging studies.