Cancer Antigen 15-3 Test

Cancer Antigen 15-3 (CA 15-3) is a protein found in the blood that is most commonly used to monitor breast cancer. Specifically, it is a type of glycoprotein, which is a molecule made up of a protein and a sugar chain. CA 15-3 comes from a larger protein called MUC1, which is normally present on the surface of many epithelial cells, the cells that line surfaces inside and outside the body. In cancer, especially breast cancer, MUC1 is produced in abnormally high amounts, leading to elevated CA 15-3 levels in the bloodstream.

Clinicians primarily use CA 15-3 to track how breast cancer is responding to treatment or to detect if the cancer is returning after treatment. In patients with advanced or metastatic breast cancer (cancer that has spread to other parts of the body), CA 15-3 levels often rise when the disease worsens and fall when it improves. This marker is particularly reliable in estrogen receptor-positive (ER-positive) and luminal types of breast cancer, where it closely mirrors the course of the disease. However, it is less reliable for triple-negative or HER2-positive breast cancers, and it does not perform well in tracking cancers that have spread only to soft tissues or the brain.

It is important to understand that elevated CA 15-3 levels do not always mean breast cancer is present or worsening. CA 15-3 can also be higher in other cancers like ovarian, lung, pancreatic, and colon cancer. Moreover, some non-cancerous conditions such as benign breast disease, liver diseases (like cirrhosis), sarcoidosis (a disease causing inflammatory lumps in the body), lupus (an autoimmune disease), pregnancy, and lactation can also raise CA 15-3 levels. In some blood cancers like myelodysplastic syndromes (MDS), CA 15-3 may be elevated too, particularly in untreated or relapsing disease.