CA 15-3 Unit Converter

CA 15-3 (cancer antigen 15-3) is a protein produced by normal breast cells and found in the blood. It belongs to the MUC1 family of mucin glycoproteins, which help protect and lubricate cell surfaces throughout the body.

When breast cancer is present, tumor cells can overproduce and shed CA 15-3 into the bloodstream, causing levels to rise. The marker is elevated in approximately 75-80% of people with metastatic breast cancer, but in fewer than 50% of those with early-stage disease.

CA 15-3 is primarily used to monitor treatment response and detect recurrence in breast cancer patients, not for screening or diagnosis. The related test CA 27.29 measures the same protein antigen.

CA 15-3 is primarily produced by breast cells, with smaller amounts made by cells in the ovaries, lungs, and other tissues. When cancerous tumors are present, they shed higher amounts of this protein into the bloodstream.