What Is the Complement System?
The complement system is a group of proteins that form part of the immune response, helping to destroy pathogens and clear immune complexes. C3 and C4 are the most commonly tested components.
Normal C3 & C4 Ranges
| Test | Normal Range |
|---|---|
| C3 | 90–180 mg/dL |
| C4 | 16–47 mg/dL |
Causes of Low Complement
- Active systemic lupus erythematosus (SLE) — immune complexes consume complement
- Lupus nephritis — low C3/C4 + high anti-dsDNA predicts kidney involvement
- Post-streptococcal glomerulonephritis — low C3 specifically
- Hereditary angioedema — low C4
- Subacute bacterial endocarditis
- Hepatitis C-associated cryoglobulinaemia
Causes of High Complement
Elevated complement is less clinically significant — it occurs as an acute-phase reaction in inflammation, infection, or obstructive jaundice.
Lupus Flare Pattern
A classic lupus flare pattern: rising anti-dsDNA antibodies + falling C3/C4 + symptoms. This combination often guides decisions to escalate immunosuppression.
FAQs
Is fasting required for C3/C4?
No fasting needed.
How often is complement tested in lupus?
Typically every 3–6 months when stable, and during flares. More frequent monitoring during nephritis.
Can complement be normal in lupus?
Yes — some lupus patients never show low complement. C3/C4 is most useful when combined with anti-dsDNA and clinical assessment.
Medical Disclaimer: Complement levels must be interpreted alongside anti-dsDNA, clinical symptoms and kidney function tests by a rheumatologist.