Canadian Cancer Society logo
You are here: 

Cancer antigen 15-3 (CA15-3)

Cancer antigen 15-3 (CA15-3) is a protein produced by a variety of cells, particularly breast cancer cells.

Why a CA15-3 test is done

A CA15-3 test may be done to check:

  • a person’s response to treatment for breast cancer
    • This is the main use of CA15-3 tests.
  • if breast cancer has come back (recurred) after treatment

Back to top

How a CA 15-3 test is done

A CA15-3 test is usually done in a private laboratory or hospital laboratory. No special preparation is usually needed.

  • CA15-3 is usually measured by a blood test.
  • The sample is sent to a laboratory to be analyzed by special machines.

Back to top

What the results mean

An increased CA15-3 value can occur in non-cancerous and cancerous conditions.

  • non-cancerous conditions
    • pregnancy
    • lactation
    • endometriosis
    • pelvic inflammatory disease
    • benign breast or ovarian disease
    • liver diseases such as cirrhosis or hepatitis
  • cancerous conditions
    • breast cancer
      • CA15-3 is often increased in advanced breast cancer or when cancer has spread (metastasized).
      • It is rarely increased in women with early stage breast cancer.
    • lung cancer
    • ovarian cancer
    • pancreatic cancer
    • liver cancer
    • stomach cancer

In cancerous conditions:

  • A decrease or return to normal value may mean that the cancer has responded well to treatment.
  • An increase may mean that the cancer is not responding well to treatment, is still growing or is coming back (recurring).
    • A slight increase may not be significant. The doctor looks at trends in the increase over time.

Back to top

What happens if a change or abnormality is found

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

Back to top


Harold Johns His amazing career and legacy live on today, inspiring a new generation of scientists who are discovering new ways to harness the power of medical imaging to improve cancer diagnosis and treatment.

Read Harold's story

Support from someone who has ‘been there’

Illustration of conversation

The Canadian Cancer Society’s peer support program is a telephone support service that matches cancer patients and their caregivers with specially trained volunteers.

Learn more