A CA 15.3 test measures the protein Cancer Antigen 15-3 in the blood, primarily used as a breast cancer marker to monitor treatment effectiveness, detect recurrence, and assess disease progression, especially in advanced stages, though it’s not for initial diagnosis due to other conditions also raising levels (like liver issues or endometriosis). Higher levels often signal worsening cancer, while decreasing levels suggest treatment is working, requiring results alongside other tests for a complete picture