CA 15.3 (breast) Test

Cancer antigen 15-3 (CA15-3) is a protein made by a variety of cells, particularly breast cancer cells. The protein moves into the blood, where it can be measured. CA15-3 levels are higher than normal in most women with breast cancer that has spread to other parts of the body (called metastatic breast cancer). Not all types of breast cancer will cause CA 15-3 levels to rise, as some types of cancer cells don’t over-produce the antigen.