Cancer antigen 15-3 (CA15-3) test

The cancer antigen 15-3 (CA15-3) test measures the amount of CA15-3 in the blood.

CA15-3 is a protein that is normally found in the body. It is mainly made by and found on the surface of breast cells. Other types of cells also make small amounts of CA15-3, such as cells in the ovaries and lungs. Some breast cancer cells and other types of cancer cells also make CA15-3. When cells break down, they release CA15-3 into the blood.

CA15-3 levels can be higher than normal with certain non-cancerous (benign) conditions. They can also be higher with certain types of cancer, especially advanced breast cancer that has spread to nearby tissues (locally advanced) or to other parts of the body (metastatic).

Because the level of CA15-3 is often high with advanced breast cancer, CA15-3 is commonly used as a tumour marker for this type of cancer. Tracking changes in CA15-3 levels helps doctors check if treatment for advanced breast cancer is working.

If you are diagnosed with advanced breast cancer, your healthcare team may order a CA15-3 test, and other tumour marker tests, to help plan your treatment. Find out more about hormone receptor status testing and HER2 status testing.

Doctors mainly use CA15-3 tests to check if advanced breast cancer is responding to treatment or to see if it has come back (recurred) after treatment. They will use this test to measure the CA15-3 level in the blood before, during and after treatment.

This test can’t be used to screen for or diagnose breast cancer for these reasons:

• Early-stage breast cancer usually doesn’t increase the level of CA15-3 in blood.
• Some types of breast cancer cells don’t make CA15-3, so they will not increase the level of this protein in the blood.
• CA15-3 levels can be higher than normal with certain non-cancerous conditions, such as endometriosis.
• Other types of cancer can increase CA15-3 levels.

A CA15-3 test is a blood test. A nurse or technologist will use a needle to take a sample of blood from a vein in your arm.

The blood is usually collected at a lab or hospital. You usually don’t need to do anything to prepare for this test.

You may have several CA15-3 tests. Doing these tests over a period of time (called serial measurements) allows your healthcare team to check if cancer treatment is working, if cancer is growing or if the cancer has come back.

Talk to your healthcare team about the results of a CA15-3 test and what they mean to you. Your doctor may recommend other tests or procedures, treatment or follow-up care.

The following non-cancerous conditions can increase the level of CA15-3 in the blood:

• endometriosis
• fibrocystic breast disease
• liver disease
• ovarian cysts
• pelvic inflammatory disease
• pregnancy and lactation

CA15-3 levels are often higher than normal when breast cancer has spread to other parts of the body. The level is usually highest when breast cancer has spread to the bones, the liver or both.

These types of cancer can also increase the level of CA15-3:

• colon
• lung
• ovary
• pancreas
• prostate

If your healthcare team is using a CA15-3 test to check if treatment for advanced breast cancer is working, they will compare CA15-3 levels before, during and after treatment.

• If the CA15-3 level decreases or returns to normal, it may mean that cancer is responding well to treatment.
• If the CA15-3 level doesn’t change or increases, it may mean that the cancer is not responding well to treatment, is still growing or has come back.

The CA15-3 level alone is not enough to confirm if treatment is working. Your healthcare team will review the CA15-3 levels along with the results of physical exams, x-rays and other imaging tests to check how advanced breast cancer is responding to treatment. Find out more about physical exam and x-ray.

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