CA15.3 is a breast cancer marker test that measures the level of a protein called Cancer Antigen 15-3 (CA15-3) in the blood. CA15-3 is produced by certain types of breast cancer cells, and its level in the blood can be used to monitor the progression of the disease in patients who have been diagnosed with breast cancer. However, it is important to note that the CA15-3 test is not used for breast cancer screening or diagnosis, and a high level of CA15-3 does not necessarily indicate the presence of breast cancer. The test is typically used in conjunction with other diagnostic tools, such as mammograms, biopsies, and physical exams, to help doctors monitor the progression of breast cancer and adjust treatment plans accordingly.