CA15-3 is a blood test that measures the level of a protein called cancer antigen 15-3. This protein is often elevated in people with certain types of cancer, particularly breast cancer. The test is used to monitor the effectiveness of breast cancer treatment and to detect recurrence. It is a simple and non-invasive test that can provide important information for cancer patients and their doctors. Overall, the CA15-3 blood test plays a valuable role in the diagnosis and management of breast cancer.