What is being tested?Cancer antigen 15-3 (CA 15-3) is a normal product of breast cells; it is produced by a
gene that is often overexpressed (i.e., the body makes too many copies) in cancerous breast
tumors, leading to an increased production of CA 15-3 and the related Cancer antigen 27.29 (which measures the same marker but in a different way). CA 15-3 does not cause cancer; rather, it is a protein that is shed by the tumor cells, making it useful as a
tumor marker to follow the course of the cancer.
CA 15-3 is elevated in about 30% of women with localized breast cancer and in about 75% of those with metastatic breast cancer (cancer that has spread to other organs). CA 15-3 also may be elevated in healthy people and in individuals with other cancers, conditions, or diseases, such as colorectal cancer, lung cancer, cirrhosis, hepatitis, and benign breast disease.
