Breast Cancer Marker Monitoring - Cancer Antigen 15-3 (CA 15-3) Test

The serum cancer antigen 15-3 (CA 15-3) test measures the level of a membrane-associated glycoprotein known as MUC1. This protein is overexpressed on the surface of cancer cells in many tumors, particularly breast cancer. Its blood levels are higher in advanced (metastatic) breast cancer than in early stages. It is not used for initial diagnosis or general screening, but primarily to monitor patients with metastatic breast cancer and assess their response to treatment.