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Blood Test Tracker

CA 15-3 (Cancer Antigen 15-3)

A protein on breast and other gland cells. Doctors use it to follow breast cancer treatment, and many harmless conditions can raise it too.

AI-verified against cited sources. Awaiting expert reviewAlso called CA 15-3, CA15-3LOINC 6875-9

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What it is

CA 15-3 is a form of a protein called MUC1 that sits on the surface of breast and other gland cells. Small amounts normally reach the blood. Breast cancer cells that have spread can release more of it. Some harmless conditions can raise it as well. A related test, CA 27.29, measures a similar protein.

Why it matters

CA 15-3 is used mostly in people who already have breast cancer. Along with scans and exams, it can help show whether treatment is working when the cancer has spread. It is not a test for finding breast cancer: levels are rarely raised in early or localized disease, and harmless conditions can raise them. The American Society of Clinical Oncology (ASCO, 2013) does not recommend tumor marker tests, including CA 15-3, for routine follow-up after treatment aimed at cure in people with no symptoms and a normal exam.

If it’s high

A result above your lab's range does not mean you have cancer or that cancer has come back. Liver conditions, benign breast disease, lung conditions and some autoimmune diseases can all raise CA 15-3. In a person who is being treated for breast cancer, clinicians watch the trend over several tests, along with imaging and symptoms.

Common causes

  • Long-term liver disease, such as chronic hepatitis or cirrhosis
  • Benign (non-cancer) breast disease
  • Pelvic inflammatory disease or endometriosis
  • Autoimmune conditions such as lupus
  • Lung conditions such as tuberculosis, sarcoidosis or interstitial lung disease
  • Late pregnancy (reported in some studies)
  • Breast cancer, especially when it has spread
  • Other cancers, such as lung, colon, pancreas, liver, ovary, cervix or uterus

What can help

  • Ask your clinician whether to repeat the test, and compare it with earlier results from the same lab
  • Tell your clinician about liver, lung or autoimmune conditions, pregnancy, and any biotin supplements you take
  • Ask how this result fits with your scans and exams
  • Use the same lab and method for follow-up tests

Usual ranges

What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.

For adults: 0–30 U/mL. Source: Mayo Clinic Laboratories, ARUP Laboratories, Quest Diagnostics, Labcorp.

What can affect your result

  • Assay method: results differ between labs and can't be compared directly
  • Liver disease and several other non-cancer conditions
  • Pregnancy: some studies found higher levels in the third trimester, and others found no change
  • Stage of disease: levels are rarely raised in early or localized breast cancer
  • Biotin supplements can interfere with some methods
  • High-dose biotin (vitamin B7, found in many hair, skin and nail supplements) can interfere with some immunoassays. Mayo asks people to avoid biotin for 12 hours before the draw, and Labcorp asks for at least 72 hours.
  • Liver disease, benign breast disease, lung conditions and some autoimmune and infectious diseases can raise CA 15-3 without cancer. Some studies also found higher levels in late pregnancy.
  • Results from different methods can't be compared directly. In College of American Pathologists survey data, slopes between CA 15-3 and CA 27.29 methods ranged from 0.836 to 1.095, mostly because of differences in calibration.
  • CA 15-3 and CA 27.29 are related but not interchangeable tests.

When to talk to a clinician

Talk with your clinician soon if you are being followed for breast cancer and your CA 15-3 is rising on repeat tests, or if you notice a new breast lump or skin change, bone pain that doesn't go away, new shortness of breath, or unexplained weight loss. If you have never had breast cancer and this test was ordered for another reason, a result above range is worth discussing, but it is most often explained by something harmless.

Questions you could ask

  • Why was CA 15-3 ordered for me, and how will we use the result?
  • Should I use the same lab each time so the results can be compared?
  • Could a liver, lung, autoimmune or other non-cancer condition explain this number?
  • Should we repeat it, and compare it with my earlier results?
  • How does this result fit with my scans and exams?

Questions people ask

Can CA 15-3 tell me if I have breast cancer?

No. It is not used to find breast cancer. Levels are rarely raised in early disease, and harmless conditions such as liver disease can raise them. Tools such as mammograms and exams are used for that. If you have been treated for breast cancer, your care team decides whether CA 15-3 is useful in your case.

Are CA 15-3 and CA 27.29 the same test?

They measure related proteins from the same gene, and doctors use them in much the same way, but the numbers aren't interchangeable. Use the same test and the same lab if you are following your results over time.

Why does my lab's range differ from another lab's?

Each lab uses its own method and cutoff. Labcorp prints 0.0 to 25.0 U/mL, Mayo Clinic Laboratories prints under 30, ARUP prints 0 to 31 and Quest prints under 32. Use the range printed on your own report and ask your clinician how to read it.

Show the science and sources

Units

  • 1 U/mL = 1 U/mL
  • 1 U/mL = 1 kU/L

Is a change real?

CA 15-3 naturally varies about 4.4% from test to test in the same healthy person. Between two results from the same lab, a rise of 15% or more, or a fall of 13% or more, is unlikely to be day-to-day noise or lab variation (95% reference change value). Source: Critical review and meta-analysis of biological variation estimates for tumor markers (Marques-Garcia F et al., EFLM WG-BV).

For clinicians and researchers

CA 15-3 is a MUC1 gene-derived glycoprotein detected by sandwich assays. The related CA 27.29 (BR 27.29) assays measure a slightly different part of the MUC1 tandem-repeat molecule, and the values are not interchangeable (Klee and Schreiber 2004). In College of American Pathologists survey data from 10 methods, the within-method medians correlated well (r above 0.99) but regression slopes ranged from 0.836 to 1.095, and much of the difference was calibration rather than antigen specificity. Duffy 2010 describes CA 15-3 as the most widely used serum marker in breast cancer. Its main use is monitoring therapy in metastatic disease, always alongside imaging, clinical history and examination; it is particularly useful when disease can't be measured by imaging. Serial postoperative testing can give a median lead time of 5 to 6 months before recurrence is clinically evident, but it is unclear whether acting on that lead time improves outcome, and expert panels disagree. Levels are rarely raised in early or localized disease. ASCO 2013 does not recommend CEA, CA 15-3 or CA 27.29 for routine follow-up of asymptomatic patients with a normal exam. NACB 2008 says CA 15-3 or BR 27.29 may be used for therapy monitoring in advanced disease. The FDA-cleared Elecsys CA 15-3 II assay is indicated as an aid in the early detection of recurrence in previously treated stage II and III breast cancer and for monitoring response to therapy in metastatic disease (FDA 510(k) summary). Non-malignant causes of raised CA 15-3 include liver cirrhosis and chronic hepatitis, tuberculosis, benign breast disease, pelvic inflammatory disease, endometriosis, lupus and sarcoidosis, and interstitial lung disease. The discussion in a 2019 case report cites a study of 350 normal pregnancies with a median CA 15-3 of 26.0 U/mL in the third trimester compared with 14.0 and 15.0 U/mL in the first and second trimesters. Reference limits differ by lab: Labcorp 0.0 to 25.0 U/mL, Mayo under 30 U/mL and ARUP 0 to 31 U/mL (all electrochemiluminescence methods; Mayo and ARUP name the Roche assay), and Quest under 32 U/mL (immunoassay). Biotin: Labcorp advises stopping at least 72 hours before collection and Mayo 12 hours.

LOINC 6875-9 (primary), 83083-6. Specimen: serum.

Sources

  1. Cancer Antigen (CA) 15-3 (test 143404), Labcorp
  2. CA 15-3 (test code 5819), Quest Diagnostics
  3. CA153 - Overview: Cancer Antigen 15-3 (CA 15-3), Serum, Mayo Clinic Laboratories
  4. Cancer Antigen-Breast (CA 15-3), test 0080464, ARUP Laboratories
  5. Elecsys CA 15-3 II: FDA 510(k) summary K171605, Roche Diagnostics (FDA 510(k) database) (2018)
  6. Breast cancer follow-up and management after primary treatment: American Society of Clinical Oncology clinical practice guideline update, Journal of Clinical Oncology (2013)
  7. CA 15-3: uses and limitation as a biomarker for breast cancer, Clinica Chimica Acta (2010)
  8. MUC1 gene-derived glycoprotein assays for monitoring breast cancer (CA 15-3, CA 27.29, BR): are they measuring the same antigen?, Archives of Pathology & Laboratory Medicine (2004)
  9. Can we trust tumour markers in pregnancy after breast cancer? A case of elevated CA 15-3 in the third trimester of pregnancy normalising after delivery, ecancermedicalscience (2019)
  10. Hypermetabolic Pulmonary and Mediastinal Lesions With Elevated Cancer Antigen (CA) 15-3 and CA 27-29 in a Patient With a History of Ovarian and Breast Cancer, Cureus (2024)
  11. National Academy of Clinical Biochemistry laboratory medicine practice guidelines for use of tumor markers in testicular, prostate, colorectal, breast, and ovarian cancers, Clinical Chemistry (2008)
  12. Tumor Marker Tests in Common Use, National Cancer Institute (2023)

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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