Complement C3 Protein (Mass Concentration)
This test measures the amount of C3, a key protein in your immune system’s complement pathway, which helps fight infections and inflammation.
["Understanding your C3 protein level can help assess immune function and response to infections or autoimmune conditions."]
Key facts at a glance
- LOINC
- 43579-2
- Short code
- C3PF
- Synonyms
- C3 Plr-mCnc
- Category
- chemistry
- Sample
- fluid
- Container
- Sterile, leak-proof container without additives; red-top tube acceptable for transport
- Units
- N/A
- Fasting
- false
- Population
- adult
- Sex
- all
- Age range
- 18–65 years
- Method
- Immunonephelometry
- CPT
- 86160
- SNOMED
- 278010007
- Specimen volume
- 1-2 mL of pleural fluid
- Stability
- Stable for 8 hours at room temperature; 24 hours at 2-8°C; 1 month at -20°C or below
- Hemolysis max
- 300
- TAT
- 4–24 hours
- Order
- routine
What Does This Test Measure?
C3 is part of the complement system—a network of proteins that enhance the immune response. It plays a role in clearing bacteria, regulating inflammation, and helping antibodies target pathogens. Low levels may indicate immune dysfunction, while high levels could suggest chronic inflammation or certain conditions like lupus.
Why Might You Need This Test?
Your doctor may order this test if you have symptoms like frequent infections, unexplained fatigue, joint pain, or signs of autoimmune disease. It’s also used to monitor conditions like kidney disease, infections, or after treatments that affect the immune system.
How Is the Test Performed?
A blood sample is drawn from a vein in your arm. The blood is processed to separate serum or plasma, where the C3 protein is measured using a lab technique (likely nephelometry or immunoassay). Results are reported in grams per liter (g/L) or milligrams per deciliter (mg/dL).
What Do the Results Mean?
Results are compared to a reference range specific to your lab, but they don’t replace a doctor’s interpretation. Very low levels may signal immune deficiencies or chronic diseases, while very high levels could indicate inflammation or certain infections. Always discuss your results with your healthcare provider.
Reference range
Reference ranges for complement C3 in pleural fluid are not well established; interpretation should be made in conjunction with serum complement levels and clinical context. Typically compared to serum C3 levels using a pleural fluid-to-serum ratio.N/A
No special patient preparation required prior to thoracentesis
Clinical interpretation
How abnormal results are interpreted and what conditions they suggest.
◆Associated findings1
Short name
C3 Protein Level
Frequently asked questions
What are common reasons for this test?
Can lifestyle changes affect C3 levels?
How often should I get this test?
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 16, 2026. · Read our editorial policy