CA 15-3 is a tumor marker commonly used in the management of breast cancer. It is a blood test that measures the level of a protein called cancer antigen 15-3 (CA 15-3) in the bloodstream. CA 15-3 is often elevated in individuals with breast cancer, particularly in those with advanced or metastatic disease. However, it is not specific to breast cancer and can also be elevated in other conditions, such as benign breast diseases and certain types of cancer.