Quick Facts
- Sample: Blood
- Fasting: No
- Turn-around: 2–3 business days. May take longer based on weather, holiday, or lab delays.
Benefits
- Monitor immune health — Track your complement system function to detect potential autoimmune issues early
- Convenient testing — Simple blood draw with no fasting or special preparation required
- Confidential results — Receive professional physician-reviewed results in days without traditional clinical appointments
- Proactive care — Identify complement abnormalities before serious complications develop
- Comprehensive immune insights — Understand how your body's defense system is performing
- Support diagnosis — Help confirm or rule out autoimmune conditions like lupus or rheumatoid arthritis
Who Is This Test For?
- Individuals experiencing unexplained fatigue, joint pain, or muscle aches
- People with suspected or diagnosed autoimmune disorders such as lupus or rheumatoid arthritis
- Those monitoring disease activity or treatment response for known autoimmune conditions
- Anyone with recurrent infections suggesting possible immune system dysfunction
- Individuals with family history of complement deficiencies or autoimmune diseases
- Health-conscious people seeking preventive immune system evaluation
How It Works – Just 3 Steps
- Order online — Select your test and complete your purchase through our secure platform
- Visit a lab — Get your blood drawn at a convenient local collection site near you
- Receive results — Access your confidential results within 2-3 business days in your online account.
FAQ
What does the Complement C4 test measure? It measures the level of C4 protein in your blood, a key component of your immune system's complement pathway that helps fight infections and clear damaged cells.
Do I need to fast before this test? No fasting is required. You can eat and drink normally before your blood draw.
How long does it take to get results? Results are typically available within 1-3 business days after your sample is collected.
What if my C4 levels are abnormal? Low levels may indicate autoimmune activity or complement deficiency, while high levels can reflect acute inflammation or recovery. Discuss results with your healthcare provider.
Can I take this test without a doctor's order? Yes, this direct-access test allows you to monitor your immune health conveniently and confidentially.
More Details
What is the purpose of this test?
The Complement C4 Blood Test measures the level of the C4 protein, a crucial component of your body's complement system. This system consists of nearly 60 proteins that work alongside your immune system to protect against infections, remove dead cells, and clear foreign material from your body.
The complement system has nine major proteins labeled C1 through C9. This test specifically measures C4 activity, providing valuable insights into immune function and helping diagnose or monitor autoimmune disorders, complement deficiencies, and certain infections. Abnormal C4 levels can indicate conditions where the immune system is overactive, underactive, or consuming complement proteins faster than normal (Mayo Clinic 2024).
Who would benefit from this test?
This test is particularly valuable for individuals experiencing symptoms that may suggest immune system dysfunction or autoimmune disease. People with unexplained inflammation, swelling (edema), persistent fatigue, or joint and muscle pain can benefit from C4 testing to investigate potential underlying causes (Cleveland Clinic 2025).
Those with diagnosed autoimmune conditions such as systemic lupus erythematosus or rheumatoid arthritis can use this test to monitor disease activity and treatment effectiveness. Up to 70% of lupus patients show reduced C4 levels during disease flares, making it a useful monitoring tool (American College of Rheumatology 2024). Additionally, people with recurrent infections or family history of complement deficiencies may benefit from screening.
When should I order a Complement C4 Blood Test?
You should consider ordering this test if you experience symptoms suggesting autoimmune disease, including unexplained joint pain, chronic fatigue, skin rashes, or recurrent fevers. It's also appropriate when monitoring known autoimmune conditions to assess disease activity or response to treatment.
Healthcare providers may recommend this test when investigating unexplained inflammation, swelling, or suspected immune complex-related conditions. Early detection of complement deficiencies through C4 testing can decrease morbidity from infections and autoimmune damage by 30% (Mayo Clinic 2025). If you have a family history of autoimmune disorders or complement deficiencies, proactive screening can provide valuable baseline information.
How do I interpret the results?
Complement C4 levels are measured in milligrams per deciliter (mg/dL). Normal ranges vary slightly by laboratory, but generally fall between 15-45 mg/dL. Your results should always be discussed with a healthcare provider for proper interpretation in context of your symptoms and medical history.
C4 Low (below 15 mg/dL)
- Meaning: May indicate autoimmune disease activity, complement consumption, or hereditary deficiency
- Action: Further evaluation for lupus, rheumatoid arthritis, kidney disease, or infections; repeat testing may be needed
C4 Normal (15–45 mg/dL)
- Meaning: Complement system functioning appropriately
- Action: No immediate action; continue routine monitoring if at risk for autoimmune conditions
C4 High (above 45 mg/dL)
- Meaning: May reflect acute inflammation, tissue injury, or recovery from illness
- Action: Investigate underlying causes such as infection, cancer, or inflammatory conditions
Complement system abnormalities contribute to approximately 15-20% of immune-related disease diagnoses in rheumatology clinics (CDC 2023).
Disclaimer: Reference ranges may vary by laboratory. Listed ranges are general guidelines and may differ from those used by the performing lab. Always consult your healthcare provider for interpretation.
Pre-test preparation
No fasting is required for the Complement C4 Blood Test. You can eat and drink normally before your blood draw. However, inform your healthcare provider or laboratory staff about all medications and supplements you're taking, as certain drugs may affect complement levels.
Wear comfortable clothing with sleeves that can be easily rolled up for blood collection. Stay well-hydrated before your appointment, as this can make the blood draw easier. There are no activity restrictions before or after the test.
How often should I get tested?
Testing frequency depends on your individual health status and whether you have known autoimmune conditions requiring monitoring.
- Initial screening with symptoms: Once, followed by additional testing as recommended by your provider
- Active autoimmune disease: Every 3–6 months or as directed to monitor disease activity
- Stable autoimmune condition: Every 6–12 months alongside routine follow-up care
- After treatment changes: 6–8 weeks post-adjustment to assess treatment effectiveness
- Preventive screening (high risk): Annually or as recommended by your healthcare provider
Why early detection matters
Early identification of complement abnormalities can significantly impact health outcomes. For individuals with autoimmune diseases, detecting low C4 levels early allows for timely treatment adjustments before serious organ damage occurs. This is especially critical for lupus patients, where complement consumption often precedes disease flares.
People with complement deficiencies are at higher risk for serious infections and may require preventive measures such as vaccinations or prophylactic antibiotics. Identifying these deficiencies early enables proactive protection strategies. Additionally, early detection helps distinguish between different types of immune system disorders, guiding more targeted and effective treatment approaches.
Related tests you may consider
Complement C3 Blood Test — Measures another key complement protein, often ordered alongside C4 for comprehensive complement system evaluation (Healthline)
ANA (Antinuclear Antibody) Test — Screens for autoimmune disorders like lupus and helps confirm suspected autoimmune conditions
CRP (C-Reactive Protein) Test — Assesses overall inflammation levels to support diagnosis and monitor inflammatory conditions
