CA 15-3 is a protein produced by many normal cells, including breast cells. Blood levels of CA 15-3 are high in 30–40% of women with a small tumour or localized breast cancer and in more than 80% of women with advanced cancer with metastases. CA 15-3 can also be high in healthy people or in people who have other cancers (intestine, lung) or diseases (cirrhosis, hepatitis, benign breast diseases).
Normal CA 15-3 levels do not guarantee the absence of breast cancer. Slightly elevated CA 15-3 levels are found in several conditions other than breast cancer. In general, the higher the CA 15-3 levels, the more advanced the cancer, and higher levels are found when metastases develop in the bones and liver. CA 15-3 levels that increase over time may indicate a cancer that is not responding to treatment or the recurrence of cancer.
Different CA 15-3 testing techniques used by laboratories can produce conflicting results. It is therefore recommended to always use the same laboratory to monitor CA 15-3 levels.