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Also reported in: g/L
Normal C4 means the classical complement pathway proteins are not being abnormally consumed or deficient. This reduces the likelihood of active immune complex-mediated complement consumption from diseases like lupus. Normal C4 alongside normal C3 makes active autoimmune complement depletion unlikely.
Very low C4 is a major finding strongly associated with autoimmune diseases, particularly systemic lupus erythematosus (SLE). C4 is heavily consumed by immune complex activation in lupus. Complete C4 deficiency (hereditary) carries a very high risk of developing lupus-like disease. Very low C4 combined with low C3 points to massive immune complex complement consumption.
Urgent immunology/rheumatology evaluation is needed. ANA, anti-dsDNA, full autoimmune panel, and kidney function should be assessed urgently.
Complement C4: {{value}} {{unit}} — very low (ref: {{low}}–{{high}}; critical: <8)Low C4 is one of the most specific laboratory clues for autoimmune disease activity. C4 is consumed when immune complexes activate the classical complement pathway, which occurs prominently in lupus, cryoglobulinemia, hereditary angioedema (where C4 is chronically depleted due to uncontrolled complement activation), and certain glomerulonephritis types. Low C4 without low C3 can suggest hereditary angioedema.
Discuss with your doctor. C3 measurement and autoimmune panel are important companion tests. Hereditary angioedema should be considered if episodes of unexplained swelling are present.
Complement C4: {{value}} {{unit}} — low (ref: {{low}}–{{high}})Borderline low C4 can reflect mild complement consumption by immune complexes, C4 gene copy number variation (low gene copies produce less C4 protein — a risk factor for lupus), or early autoimmune disease activity. C4 gene polymorphisms causing partial C4 deficiency are more common in people of certain ethnicities and are associated with increased lupus susceptibility.
Monitor alongside C3. If both are trending low, autoimmune panel evaluation is appropriate. Discuss symptom review for autoimmune features with your doctor.
Complement C4: {{value}} {{unit}} — borderline low (ref: {{low}}–{{high}}; borderline: 16–20)Normal C4 means the classical complement pathway proteins are not being abnormally consumed or deficient. This reduces the likelihood of active immune complex-mediated complement consumption from diseases like lupus. Normal C4 alongside normal C3 makes active autoimmune complement depletion unlikely.
Normal result. No complement consumption pattern detected.
Complement C4: {{value}} {{unit}} — normal (ref: {{low}}–{{high}})Optimal C4 reflects adequate complement protein availability and no significant complement consumption. The classical complement pathway, which C4 serves, is an important defense against certain bacterial infections and plays a role in clearing immune complexes safely from the body. Optimal levels support both immune defense and proper immune complex clearance.
Excellent result. Normal complement status.
Complement C4: {{value}} {{unit}} — optimal (ref: 20–45)Like C3, C4 is an acute-phase reactant that rises mildly with inflammation. A borderline elevated C4 can reflect minor inflammatory activation or be a normal upper variation. This finding is generally less clinically concerning than low C4.
Review in context of other inflammatory markers. Not typically notable as an isolated finding.
Complement C4: {{value}} {{unit}} — borderline elevated (ref: {{low}}–{{high}}; borderline: 47–60)Elevated C4 reflects increased liver synthesis of complement proteins in response to systemic inflammation or infection. This can occur with active inflammatory disease, chronic infection, or metabolic conditions stimulating acute-phase protein production. Elevated C4 is not typically the primary concern — the underlying inflammatory trigger is.
Investigate underlying inflammatory source with CRP, ESR, and clinical assessment.
Complement C4: {{value}} {{unit}} — elevated (ref: {{low}}–{{high}}; elevated: >60)Very high C4 reflects a significant acute-phase inflammatory response with marked complement upregulation. As with high C3, the complement elevation itself is a marker of the underlying inflammatory process rather than a primary disease. Investigate the underlying driver.
Comprehensive inflammatory workup is needed. Identify the underlying inflammatory trigger driving this complement upregulation.
Complement C4: {{value}} {{unit}} — very high (ref: {{low}}–{{high}}; very high: >80)Upload your lab report and get your actual values interpreted in plain English — instantly, with no medical training required.