The CA 15-3 test is a blood test used in monitoring and predicting the progression of breast cancer. CA 15-3 is a tumor marker found on the surface of breast cancer cells. This test measures the concentration of CA 15-3 in the blood. The main purpose of the CA 15-3 test is to monitor treatment response and detect recurrence of breast cancer in diagnosed patients. Elevated CA 15-3 levels may indicate disease progression or the appearance of metastases. It's important to note that CA 15-3 levels can also be elevated in some non-cancerous conditions such as liver inflammation, cirrhosis, thyroid disease, and lung disease. Therefore, the interpretation of CA 15-3 test results needs to be combined with other diagnostic methods such as clinical examination, imaging, and biopsy.