Cancer antigen 15-3 (CA 15-3) is a protein that is produced by normal breast cells. In many people with cancerous breast tumours, 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 tumour cells and enters the blood. This test measures CA 15-3 in the blood.
Since CA 15-3 can be measured in the blood, it is useful as a tumour marker to follow the course of cancer. CA 15-3 is elevated in fewer than 50% of women with early localized, breast cancer or with a small tumour, but is elevated in about 80% of those with breast cancer that has spread (metastatic). Because not all women with invasive breast cancer will have elevated CA 15-3, the test is not useful in all cases.
CA 15-3 is not recommended as a screening test to detect breast cancer in women because it is non-specific. It may also be elevated in healthy people and in individuals with other cancers such as colon, lung, pancreas, ovarian, or prostate malignancies or certain conditions such as cirrhosis, hepatitis, and benign breast disease.