Cancer Antigen (CA 15.3) test is an important tumor marker that is used to measure the amount of CA protein manufactured by especially the breast cancer cells. Women with early localized small tumor or breast cancer shows slight increase in CA- 15-3 but those with cancer in the stage of metastasis shows a higher level of this protein. Doctors recommend this test to find out the effectiveness of medications used in breast cancer treatment or the recurrence of the disease after treatment.
Patients suffering from lung, pancreas, ovarian or prostate cancer may also show increased level of CA 15-3 but this level is not higher than the breast cancer patients. To confirm the level or stage of breast cancer doctors may suggest other test like hormone receptor testing and HER2 status testing. The level of CA 15-3 is highest in breast cancer patients when the cancer has spread to liver or bones. In the early stage of breast cancer CA 15-3 test is usually not recommended as in such a case the report would show only the normal level and not elevated one.