Cancer antigen 15-3 (CA15-3) test

Cancer antigen 15-3 (CA15-3) is a protein made by a variety of cells, particularly breast cancer cells. The protein moves into the blood, where it can be measured.

CA15-3 levels are higher than normal in most women with breast cancer that has spread to other parts of the body (called metastatic breast cancer). Not all types of breast cancer will cause CA 15-3 levels to rise, as some types of cancer cells don’t over-produce the antigen.

CA15-3 is a tumour marker. It is used to check how breast cancer treatment is working and look for cancer that has come back, or recurred, after treatment.

If you are diagnosed with breast cancer that has spread to other parts of the body, or metastasized, you may have this test, along with other tests such as hormone receptor testing and HER2 status testing.

CA15-3 is not measured for early stage breast cancer because the levels of this protein are rarely higher than normal at this stage.

A CA15-3 test is a blood test done with a needle. It’s usually done in a private lab or hospital lab. You don’t need to do anything special to prepare.

CA15-3 levels can be higher than normal with cancerous and non-cancerous conditions. CA15-3 is most often increased in breast cancer that has spread to other parts of the body.

In general, the higher the level of CA15-3 in the blood, the more cancer there is in the body. The levels are highest when breast cancer has spread to the bones, the liver or both. If the level of CA15-3 goes down or returns to normal, it may mean that treatment is working. If levels increase over time, it may mean that the cancer is not responding well to treatment, is still growing or is coming back (recurring).

CA15-3 may be higher than normal in cancer of the lung, pancreas, ovary and prostate, but these levels are not as high as with breast cancer.

Non-cancerous conditions that increase CA 15-3 include endometriosis, pelvic inflammatory disease and liver disease. It can also be increased during pregnancy. With these conditions, CA15-3 levels usually only go so high. They don’t usually keep climbing over time.

The doctor will decide if more tests, procedures, follow-up care or different treatment is needed.