Complement C3 and C4
A blood test measuring two key proteins in the immune system's complement pathway to help evaluate immune function and monitor autoimmune conditions like lupus.
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LevelPanel has no business or affiliate relationship with Quest or LabCorp. These links are provided as a public service.
Lab testing services linked from this site are not available to residents of Alaska, Arizona, or Hawaii, unless otherwise noted on the lab's own website.
This publication is not intended to solicit the purchase of laboratory testing from any individual consumer.
What this test measures
This test measures the blood levels of complement proteins C3 and C4, two key components of the complement system, a network of proteins that forms part of the body's innate immune defense. Complement proteins circulate in an inactive state and, when triggered, activate in a sequential cascade that helps destroy bacteria and viruses, clear dead cells and immune complexes, and amplify inflammatory responses.
Measuring C3 and C4 together provides diagnostic insight because the two proteins participate in different activation pathways. When both C3 and C4 are decreased, it typically indicates activation of the classical complement pathway, which is often driven by immune complex formation in conditions like systemic lupus erythematosus. When C3 alone is low while C4 remains normal, activation of the alternative pathway is more likely. It is worth noting that C3 and C4 are also acute-phase reactants, meaning their production can increase during active inflammation, potentially masking consumption and producing normal-appearing levels even when the complement system is being actively depleted.
What's included
Who should consider this test
You might consider this test if you are experiencing any of the following:
- Individuals experiencing symptoms that may indicate autoimmune conditions, such as persistent joint pain and swelling, skin rashes, or unexplained fatigue, as these tests can help evaluate whether the complement system is properly functioning.
- Those with a family history of autoimmune disorders like lupus, rheumatoid arthritis, or other inflammatory conditions, since complement deficiencies can have genetic components and early detection may be beneficial.
- Patients who have recurrent bacterial infections or unexplained frequent illnesses, as complement deficiencies can impair the body's ability to fight off certain types of infections effectively.
- Individuals diagnosed with autoimmune conditions who need ongoing monitoring of disease activity, since changes in C3 and C4 levels can indicate whether treatments are working or if the condition is flaring.
- People with unexplained kidney problems or blood in urine, particularly when other autoimmune markers are being investigated, as complement abnormalities are associated with certain types of kidney disease and glomerulonephritis.
What to expect
Preparation
No fasting is required for this test. Individuals may eat and drink normally before their appointment. It's helpful to inform the healthcare provider about any medications, vitamins, or supplements being taken, as some may potentially affect test results.
Sample Type
The test requires a simple blood draw from a vein, typically from the arm. The procedure takes just a few minutes and involves inserting a small needle to collect the blood sample in special tubes.
Collection
A healthcare professional will clean the skin, insert a needle into a vein, and collect the blood sample. After collection, pressure will be applied to prevent bleeding, and a bandage may be placed over the site. No doctor visit is required --- the sample can be collected at any participating lab location.
Turnaround
Results are typically available within 1-3 business days after the sample reaches the laboratory. Results will be securely provided through the online portal with clear explanations of the findings.
Understanding your results
C3 and C4 levels are reported in milligrams per deciliter (mg/dL), each with its own reference range. The diagnostic value lies in the pattern of the two results together rather than either value alone. Decreased levels of both C3 and C4 suggest immune complex-driven complement consumption, while an isolated C3 decrease points toward alternative pathway activation. Elevated levels may reflect an acute-phase inflammatory response. Healthcare providers interpret complement levels alongside ANA, anti-dsDNA, clinical symptoms, and other immune markers to evaluate autoimmune conditions, monitor disease activity, and guide treatment decisions.
| Population | Reference Range | Notes |
|---|---|---|
| Adults | C3: 80-178 mg/dL (0.8-1.7 g/L), C4: 16-48 mg/dL (0.16-0.48 g/L) | Normal ranges may vary slightly between laboratories |
Reference ranges may vary by laboratory and individual factors. Results should be interpreted by a healthcare provider in the context of your overall health.
What does a “Low” result mean?
If C3 and C4 levels are both reduced, this may be a sign of lupus or other autoimmune conditions. Low C3 alone is often associated with infectious diseases like septicemia or endocarditis, while low C4 alone is characteristically seen in conditions like angioedema or certain immune complex diseases. Low C3 and C4 levels may also indicate alcoholic liver disease, though this is less common. Complement deficiencies can be either inherited or acquired, and low levels may indicate increased consumption due to ongoing immune system activation.
What does a “High” result mean?
High C3 complement levels may indicate inflammation in the body, which could be due to conditions such as ulcerative colitis or other acute diseases like certain cancers. Elevated levels up to twice the upper limit can be found during acute phase responses. Increased levels may also occur as part of the body's response to infection, injury, or other inflammatory processes.
Privacy & confidentiality
All results are completely confidential and protected under HIPAA regulations. Test results are not shared with insurance companies, will not appear on insurance EOBs, and are not added to personal medical records unless specifically requested. Results are securely delivered through an encrypted online portal.
Frequently asked questions
Related tests
ANA (Antinuclear Antibody)
Often ordered together to screen for autoimmune conditions like lupus
View TestBasic Autoimmune Panel
Comprehensive screening that includes complement testing along with other autoimmune markers
View TestC-Reactive Protein (CRP)
Another inflammation marker that helps assess overall inflammatory status
View TestSedimentation Rate (ESR)
Inflammation marker that may be elevated when complement levels are abnormal
View TestRelated conditions
Last reviewed: April 2026
CPT Code: 86160, 86160
This test may not be available in: NY, NJ, RI
This page is for informational purposes only and does not constitute medical advice. Always consult with a healthcare provider regarding any health concerns. LevelPanel does not diagnose, treat, or prescribe.
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LevelPanel has no business or affiliate relationship with Quest or LabCorp. These links are provided as a public service.
Lab testing services linked from this site are not available to residents of Alaska, Arizona, or Hawaii, unless otherwise noted on the lab's own website.
This publication is not intended to solicit the purchase of laboratory testing from any individual consumer.
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