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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What this test measures
The Complement C3 and C4 test measures the levels of two essential proteins in the body's complement system, which comprises more than 30 proteins that play significant roles in the immune system. The complement system includes nearly 60 proteins that are in blood plasma or on cell surfaces and work with the immune system to protect the body from infections and to remove dead cells and foreign material. These proteins function as part of the body's natural defense against disease-causing bacteria, viruses, and other harmful substances.
Measurements of serum complement components C3 and C4 are useful in the diagnosis and monitoring of immune complex disease such as systemic lupus erythematosus (SLE) and some blood-associated infectious diseases. Decreased concentrations of both C3 and C4 suggest activation of the classical pathway, whereas decreased concentration of just C3 suggests activation of the alternative pathway. Complement concentrations are acute phase proteins and may be normal despite complement consumption in some inflammatory and infective disorders. When tested together, C3 and C4 levels can help healthcare providers understand which complement pathway is being activated and potentially identify the underlying cause of immune system dysfunction.
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
These tests are useful for diagnosing and monitoring immune complex diseases such as systemic lupus erythematosus and some infectious diseases. Results help healthcare providers understand how well the complement system is functioning and whether there are signs of autoimmune activity or complement deficiency.
| 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
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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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Not sure if this is the right test? Contact our care team for guidance.