Cancer antigen 15-3 (CA 15-3) blood test
This blood test measures the levels of a protein called cancer antigen 15-3, which may be produced by certain types of cancer, including breast cancer.
["CA 15-3 helps monitor cancer progression or response to treatment in some patients."]
Key facts at a glance
- LOINC
- 109071-1
- Short code
- CA153
- Synonyms
- Cancer Ag15-3 Msmt SerPl
- Category
- chemistry
- Sample
- serum
- Container
- Serum separator tube (SST) or red-top tube; plasma may be collected in lithium heparin or EDTA tubes
- Units
- N/A
- Ref low
- 30
- Ref high
- 40
- Critical range
- 0.5 N/A – —
- Fasting
- false
- Population
- adult
- Sex
- all
- Age range
- 18–65 years
- Method
- Immunoassay (chemiluminescent or electrochemiluminescent immunoassay)
- CPT
- 86300
- SNOMED
- 764713009
- Specimen volume
- 1 mL serum or plasma (minimum 0.5 mL)
- Stability
- Room temperature: 8 hours; Refrigerated (2-8°C): 7 days; Frozen (-20°C or below): 3 months. Avoid repeated freeze-thaw cycles.
- Hemolysis max
- 100
- Lipemia max
- 500
- Icterus max
- 50
- TAT
- 4–24 hours
- Order
- routine
What Is CA 15-3 Used For?
CA 15-3 is primarily used as a biomarker to track the activity of certain cancers, particularly in patients who have already been diagnosed with breast cancer or other malignancies that may produce this protein. It can help doctors assess whether cancer is spreading or if treatment is working. However, it is not a definitive test for cancer diagnosis and should be interpreted alongside other clinical findings and tests.
How Is the Test Performed?
A blood sample is drawn from a vein, typically in your arm. The blood is then processed to separate the serum (liquid part of blood without clotting factors). The lab analyzes the serum for the presence of CA 15-3 protein using an immunoassay method.
What Does a High or Low Level Mean?
Elevated levels of CA 15-3 may suggest the presence of cancer, particularly in patients with breast cancer or other cancers that produce this marker. However, it can also be elevated in other conditions, such as liver disease, infections, or certain types of inflammation. Low levels do not necessarily mean cancer is absent, as levels can vary widely and fluctuate with health status.
Who Might Need This Test?
Doctors may order CA 15-3 testing for patients who have been diagnosed with breast cancer, those undergoing treatment for breast cancer, or individuals with other suspected cancers that may produce CA 15-3. It may also be used in screening or monitoring in certain clinical settings.
Reference range
Normal: <30 U/mL; Borderline: 30-40 U/mL; Elevated: >40 U/mL. Reference values may vary by laboratory and assay manufacturer.N/A
No fasting required. Specimen should be collected prior to administration of biotin-containing supplements (wait at least 8 hours after biotin dose). Avoid hemolysis.
Clinical interpretation
How abnormal results are interpreted and what conditions they suggest.
Clinical Significance: CA 15-3 plays a role in monitoring cancer progression and response to therapy in specific patient populations. While it can be useful for detecting recurrence or disease activity, it is not a standalone test for cancer diagnosis. Elevated levels should be evaluated in the context of a patient’s full medical history, physical examination, and other diagnostic tests. False positives and negatives can occur, so results should be discussed with your healthcare provider.
Short name
CA 15-3
Frequently asked questions
Why might my CA 15-3 level be higher than expected?
Can CA 15-3 levels change over time?
Is CA 15-3 the same as CEA (Carcinoembryonic Antigen)?
Sources
- 1
- 2Lab Tests OnlineSource
- 3
Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.
This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.
This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.
If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.
Medically reviewed by WellAlly Editorial Team Last reviewed August 17, 2026. · Read our editorial policy