CA 15-3 is a blood test used to determine the presence of the CA 15-3 tumor-associated serum marker. This serum marker is used for staging breast cancer and monitoring its treatment. The risk of breast cancer increases gradually as a woman gets older. This disease is uncommon in women under the age of 35. All women age 40 and older are at risk, for breast cancer. However, most breast cancers occur in women over the age of 50, and the risk is especially high for women over age 60, The risk factors of breast cancer are personal history of breast cancer, genetic alternations, family history, certain breast changes, breast density, radiation therapy, and late childbearing. Also at a somewhat increased for developing breast cancer are women who started menstruating at an early age (before age 12), experienced menopause late (after age 55), never had children, or took hormone replacement therapy or birth control pills for long period of the time. Each of these factors increases the amount of time a woman's body is exposed to estrogen. The longer this exposure, the more likely she is to develop breast cancer.