CA 15-3 Test
The CA 15-3 test measures serum cancer antigen 15-3, a circulating form of the MUC1 glycoprotein overexpressed by breast carcinoma. It is used exclusively for monitoring known breast cancer — tracking treatment response in metastatic disease and surveilling for recurrence after curative therapy — not for screening or diagnosis. Serial trends are interpreted with imaging and clinical context.
1
Biomarkers
Blood (serum)
Sample
2–3 business days
Turnaround
No
Fasting
What is the CA 15-3 test?
The CA 15-3 test measures serum concentrations of a glycoprotein shed by breast epithelium and overexpressed in breast carcinoma. It is ordered exclusively in patients with an established diagnosis of breast cancer, not for screening asymptomatic populations or diagnosing a breast mass. Its principal role is serial monitoring: tracking response to therapy in metastatic disease, and in some centres surveying for biochemical recurrence after curative treatment. Because only about half of patients with early-stage disease and two-thirds with metastatic disease show elevated CA 15-3 at presentation, a normal result never rules out cancer. Conversely, benign liver disease, other carcinomas, and even pregnancy can raise CA 15-3 into the abnormal range. Practitioners rely on trends rather than thresholds — a rising trajectory during treatment suggests progression, a falling trend indicates response, and a stable low concentration during remission is reassuring. Guidelines from oncology societies do not recommend CA 15-3 for routine surveillance after curative surgery, reserving it for high-risk cohorts or metastatic management, so the decision to order rests on the oncologist's judgement of how the result will alter management.
What does the CA 15-3 test measure?
Cancer antigen 15-3 is a circulating form of MUC1, a large transmembrane glycoprotein expressed on the apical surface of secretory epithelium. In normal physiology MUC1 is shed into secretions and only trace amounts reach the circulation. Malignant transformation of breast epithelium leads to overexpression, aberrant glycosylation, and loss of polarity, releasing larger quantities of the glycoprotein into serum. CA 15-3 is measured by immunoassays recognising epitopes on the extracellular tandem-repeat domain; the protein's half-life is short, and serum concentrations reflect both tumour burden and the rate of antigen release.
CA 15-3 is used exclusively for monitoring known breast cancer; it is neither sensitive nor specific enough for screening or primary diagnosis. Elevated concentrations correlate with advanced disease, and serial measurements track treatment response or detect recurrence in patients with metastatic breast cancer. Rising values during surveillance may precede clinical or radiological evidence of progression by weeks to months, prompting imaging. A single result interpreted in isolation is of little value — the trend over time is the clinical signal.
Why is the CA 15-3 test ordered?
- Monitoring response to systemic therapy in patients with metastatic or recurrent breast cancer, where serial values complement imaging
- Surveillance for biochemical recurrence in selected high-risk patients after curative treatment, when early detection of metastatic disease may change management
- Investigation of suspected disease progression when imaging is equivocal or symptoms are non-specific and a rising trend would prompt further work-up
- Baseline measurement before initiating therapy for metastatic breast cancer, establishing the patient's own reference for subsequent comparison
- Differentiation of breast cancer from other malignancies in a patient with an unknown primary and elevated tumour markers, interpreted with imaging and histology
- Assessment of treatment efficacy in neoadjuvant or adjuvant settings in clinical trial protocols, though not standard-of-care outside trials
- Correlation with other breast tumour markers such as CA 27.29 or CEA when a multi-marker panel is used for surveillance
Sample collection and turnaround
Sample type
Blood (serum)
Fasting required
No
Collection method
Venipuncture at patient service centre
Turnaround
2–3 business days
Collection notes
Phlebotomy at the processing laboratory's draw site (Quest or Access Medical Laboratories, depending on routing); no special preparation. Time the draw relative to the treatment cycle if monitoring response, and note the assay platform for serial comparability.
Specimen requirements
Minimum 0.5 mL serum (gel separator acceptable). Separate from cells promptly; CA 15-3 is stable refrigerated for up to 72 hours or frozen for longer storage. Record the performing laboratory for serial comparison, as reference intervals and assay calibration differ.
What can affect CA 15-3 results?
Chronic liver disease, cirrhosis, and hepatitis can raise CA 15-3 through altered glycoprotein clearance and non-malignant epithelial turnover; correlation with liver enzymes and imaging helps separate hepatic from oncological causes.
Lung, ovarian, pancreatic, and colorectal carcinomas can express MUC1 and elevate CA 15-3; the marker is breast-associated but not breast-specific, so clinical context and imaging are essential.
Normal breast epithelial activity during pregnancy and lactation can modestly raise CA 15-3; the marker should not be ordered for cancer surveillance in pregnant or breastfeeding women without careful clinical correlation.
Different immunoassays use distinct antibody pairs and calibration, producing results that are not directly interchangeable; serial measurements should be performed on the same platform and interpreted with the same reference interval.
Systemic inflammation, autoimmune disease, and benign breast conditions such as mastitis can transiently raise CA 15-3; repeat measurement after resolution helps confirm a malignant source.
CA 15-3 may transiently rise in the first weeks of effective chemotherapy or radiotherapy owing to tumour lysis and antigen release (a 'flare' phenomenon); interpretation requires correlation with clinical and radiological response.
What do CA 15-3 results mean?
Elevated
Metastatic or advanced breast cancer (most common), recurrent disease after curative treatment, benign liver disease, other epithelial malignancies (lung, ovarian, pancreatic, colorectal), pregnancy or lactation, and inflammatory or autoimmune conditions. Values above 100 U/mL strongly suggest metastatic disease, though the threshold varies by assay.
Normal
A normal CA 15-3 does not exclude breast cancer — sensitivity is low in early-stage disease and only modest even in metastatic disease. It is used to track known malignancy, not to rule it out, and a normal baseline does not prevent serial monitoring if the clinical picture warrants it.
Reduced
Falling CA 15-3 during treatment reflects tumour response and reduction in antigen-secreting burden. A decline of ≥25–50% from baseline is generally considered a biochemical response, though no universally standardised threshold exists and trends are interpreted with imaging.
Note: CA 15-3 must never be used for screening or diagnosis; it is a monitoring tool only. Interpret serial values as a trend rather than isolated results, and always correlate with imaging, clinical examination, and the patient's treatment history. Assay platform matters — ensure continuity of testing laboratory for longitudinal comparison.
Frequently asked questions
No. CA 15-3 is neither sensitive nor specific enough for screening; it is elevated in only about half of early-stage breast cancers and can be raised by benign conditions. It is used solely for monitoring patients with an established diagnosis.
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. Tumor markers in breast cancer: EGTM recommendations. Br J Cancer. (2017)
- National Comprehensive Cancer Network. NCCN Clinical Practice Guidelines in Oncology: Breast Cancer. (2023)
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.