Autoimmune serology
Complement C3 / C4
Acute-phase complement proteins; consumed in immune-complex disease.
Sample
Serum
Clinical reference range
90 to 180 mg/dL
C3 adult · C4 10–40 mg/dL
Conv: 90–180 mg/dL
Biology
What it measures
Central proteins of the complement cascade. Hepatically synthesized; consumed when activated by immune complexes.
Clinical use
Why we order it
SLE flare (low C3/C4), post-strep GN, MPGN, cryoglobulinemia, hereditary angioedema (low C4).
Differential
Causes of abnormal values
Causes of LOW
- ↓SLE flare
- ↓Post-streptococcal GN
- ↓Cryoglobulinemic vasculitis
- ↓MPGN
- ↓Hereditary angioedema (C4 only)