The CA-15.3 test is an important tool for managing breast cancer. It is a 'tumor marker' test that measures the level of a specific protein in the bloodstream. This test is not a screening tool for the general public. Instead, it is an invaluable test for doctors to monitor a patient's response to therapy for advanced breast cancer and to watch for the recurrence of the disease after treatment. A simple blood draw is all that's required. This test provides a vital, quantitative number that helps your oncology team make the most informed decisions about your ongoing care.