Cancer Antigen (CA) 15-3
The CA 15-3 test measures serum concentration of a mucin glycoprotein fragment shed by breast epithelial cells and released in increased amounts by breast carcinoma. It is used to monitor disease activity and treatment response in patients with metastatic breast cancer, not for screening or diagnosis, with rising trends often preceding radiological progression.
1
Biomarkers
Blood (serum)
Sample
1–3 business days
Turnaround
No
Fasting
What is the CA 15-3 test?
The Cancer Antigen 15-3 test measures serum concentration of a mucin glycoprotein fragment shed by breast epithelial cells and released in increased amounts by breast carcinoma. It is ordered to monitor patients with metastatic breast cancer for disease progression or response to treatment, not to screen for or diagnose new disease. CA 15-3 has no role in population screening or in the diagnostic work-up of a breast mass, because its sensitivity is too low in early-stage disease and benign conditions can produce false elevations. Its value lies in serial measurement: a rising trend often precedes radiological progression by weeks to months, prompting earlier imaging or a change in therapy, while a falling or stable level during treatment supports continuation of the current regimen. Practitioners rely on the marker as one element of a broader surveillance strategy that includes clinical examination and periodic imaging rather than as a stand-alone decision tool.
What does the CA 15-3 test measure?
Cancer antigen 15-3 is a circulating mucin glycoprotein fragment shed from the surface of epithelial cells, particularly those of mammary origin. It is the product of the MUC1 gene and represents part of the extracellular domain that is cleaved and released into the bloodstream. Low concentrations are detectable in healthy individuals and in benign breast and liver conditions; substantial elevation occurs when large tumour burdens or metastatic disease drive increased shedding into the circulation.
CA 15-3 is used to monitor disease activity and treatment response in patients with established metastatic breast cancer. It is not recommended for screening, diagnosis, or staging of early breast cancer because sensitivity and specificity are inadequate in those settings. Serial measurement can detect biochemical progression or response months before imaging changes, supporting decisions to continue or alter systemic therapy, but results must always be integrated with clinical and radiological findings rather than dictating management in isolation.
Why is the CA 15-3 test ordered?
- Monitoring of patients with metastatic or recurrent breast cancer during systemic therapy to assess treatment response and detect early progression
- Baseline measurement at the time metastatic disease is diagnosed, to establish an individual reference for subsequent serial comparison
- Evaluation of suspected recurrence in patients with a history of breast cancer and new symptoms, integrated with imaging and clinical assessment
- Serial follow-up after completion of primary treatment in selected higher-risk patients, though this practice is debated and not universally recommended by guidelines
- Correlation of marker trends with imaging findings when a discrepancy exists between biochemical and radiological response
- Research or clinical-trial settings where standardised biochemical endpoints are required for response assessment
Sample collection and turnaround
Sample type
Blood (serum)
Fasting required
No
Collection method
Venipuncture at patient service centre
Turnaround
1–3 business days
Collection notes
Phlebotomy at LabCorp patient service centre; no special preparation required. Serial measurements should be performed with the same assay platform when possible for trend comparability.
Specimen requirements
Minimum 0.5 mL serum (gel separator acceptable). Separate from cells promptly. CA 15-3 is stable refrigerated for up to one week; freeze for longer storage. Note the assay method and performing laboratory for serial consistency.
What can affect CA 15-3 results?
Mastitis, fibrocystic change, and hepatitis or cirrhosis can mildly raise CA 15-3, so an elevated result without known malignancy requires clinical and imaging correlation before attributing it to cancer.
Ovarian, lung, pancreatic, and gastrointestinal carcinomas can shed MUC1 and raise CA 15-3, so the marker is not specific to breast cancer and must be interpreted in the context of the patient's known or suspected primary tumour.
Different immunoassay platforms and calibrators produce results that are not directly interchangeable; serial measurements should ideally be performed in the same laboratory with the same method.
Physiological changes in breast epithelium during pregnancy and breastfeeding can modestly increase CA 15-3, though this is rarely a clinical confounder in the oncology population.
Initiation of effective systemic therapy or local treatment can produce a temporary spike in CA 15-3 due to tumour lysis or cell death, so an early rise should not be misinterpreted as progression without clinical and imaging context.
Endogenous antibodies or biotin supplementation can cause spurious results in some immunoassay formats; discordant results warrant repeat testing after an interval or with an alternative method.
What do CA 15-3 results mean?
Elevated
In a patient with known metastatic breast cancer, elevated CA 15-3 suggests active disease, and a rising trend over serial measurements indicates progression. In newly diagnosed metastatic disease a high baseline carries prognostic information but does not alter staging. In patients without known malignancy, modest elevation is non-specific and may reflect benign breast or liver disease, other epithelial malignancies, or assay interference; imaging and clinical assessment are required.
Normal
Normal CA 15-3 does not exclude breast cancer — sensitivity is poor in early-stage disease and a substantial minority of metastatic patients do not shed sufficient antigen to produce elevation. The marker should never be used in isolation to rule out malignancy or progression; clinical and radiological assessment take precedence.
Reduced
A falling CA 15-3 during treatment reflects tumour response and is one marker of therapeutic efficacy. A decline to within the reference range is favourable but does not guarantee complete remission, so imaging remains essential. Persistently low or normal values in a patient with known metastatic disease suggest either low-volume or marker-negative disease.
Note: CA 15-3 must be interpreted by trend and in the context of the patient's treatment history, imaging, and clinical status. An isolated measurement has limited value. Guidelines do not recommend routine CA 15-3 surveillance in all breast-cancer survivors because evidence that early biochemical detection of recurrence improves survival is lacking; its role is most established in monitoring known metastatic disease. Always correlate with imaging before concluding that biochemical progression mandates a change in therapy.
Frequently asked questions
No. CA 15-3 is not recommended for screening or early diagnosis because its sensitivity is too low in early-stage disease and benign conditions can produce false elevations. It is used to monitor patients with known metastatic breast cancer.
How to access the CA 15-3 test
The CA 15-3 reference page is free and open to all practitioners. Create a free practitioner account to see pricing, save tests to your list, and generate requisition forms. Verification takes 1–2 business days.
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Published: October 7, 2026
Last reviewed: October 7, 2026
- Harris L, et al. American Society of Clinical Oncology 2007 update of recommendations for the use of tumor markers in breast cancer. J Clin Oncol. (2007)
- Cardoso F, et al. ESO-ESMO international consensus guidelines for advanced breast cancer (ABC 5). Ann Oncol. (2020)
- Duffy MJ, et al. Tumour markers in breast cancer: European Group on Tumour Markers recommendations. Eur J Cancer. (2017)
- NICE guideline [NG101]: Early and locally advanced breast cancer: diagnosis and management. (2018)
This content is for licensed healthcare practitioners only and does not constitute medical advice. Clinical decisions should be based on the full clinical picture and not on any single test result.