Cancer antigen 15-3 (CA 15-3) is a protein that is produced by normal breast cells. In many people with cancerous breast tumors, there is an increased production of CA 15-3 and the related cancer antigen 27.29. CA 15-3 does not cause cancer; rather, it is shed by the tumor cells and enters the bloodstream, making it useful as a tumor marker to follow the course of the cancer.
CA 15-3 is elevated in only about 10% of women with early localized breast cancer but is elevated in about 80% of those with metastatic breast cancer.
CA 15-3 may also be elevated in healthy people and in individuals with other cancers (e.g., colon, lung, pancreas, ovary, or prostate malignancies) or certain conditions (e.g., cirrhosis, hepatitis, and benign breast disease).