Interpret Your Complement C4 Results
Enter your C4 value below and get an instant, free AI interpretation — a plain-English explanation checked against WHO/IFCC reference ranges.
Medical Disclaimer
This AI-generated interpretation is for educational purposes only and does not constitute medical advice, diagnosis, or treatment. Results may vary based on individual health factors not captured here.
Always consult a qualified healthcare professional before making any health decisions. Read full disclaimer →
Complement C4 Normal Ranges
C4 (Complement component 4) is an early-pathway complement protein involved in immune complex clearance.
| Age Group | Reference Range | Unit | Notes |
|---|---|---|---|
| Adults (18–64) | 16 – 47 | mg/dL | — |
Also reported in: g/L.
Why Doctors Order the Complement C4 Test
C4 null alleles are common in the general population — approximately 35% of people carry at least one C4 null allele, which causes constitutional low C4 without any disease. This is an important source of false "low C4" results.
How to Enter Your C4 Result
Complement C4 can be reported in mg/dL or g/L. Choose the unit printed next to your result on the lab report so it is compared with the matching reference range.
What a Low or High C4 Can Be Associated With
Below the reference range
- Active SLE / lupus nephritis (most common clinical cause — immune complex consumption)
- Hereditary angioedema types 1 and 2 (C1-inhibitor deficiency — C4 is low even between attacks)
- C4 null alleles (genetic — no disease, just constitutional low C4)
- Infective endocarditis (subacute)
- Cryoglobulinemia (often hepatitis C-related)
Above the reference range
- Acute phase response (C4 is a mild acute phase reactant)
These are common associations, not a diagnosis. A single result is read alongside your symptoms, history and other tests — discuss it with your doctor.
What Can Change Your C4 Result
Before you read too much into a result, consider whether any of these applied:
- C4 null alleles (constitutional low C4 in ~35% of population — very common source of low C4 results)
- Sample degradation if left at room temperature
- Lab variability between assay methods
Preparing for the test: No fasting required. Process sample promptly.
Symptoms That Often Prompt C4 Testing
- In lupus: malar rash, joint pain, kidney disease
- In hereditary angioedema: recurrent episodes of deep tissue swelling (face, throat, gut, genitalia) without hives — can be life-threatening
- HAE attacks often triggered by minor trauma, stress, or estrogen (oral contraceptives)
Want the full clinical guide to Complement C4?
Reference ranges, causes, symptoms, preparation, and FAQs.