Classical Pathway Complement Marker
C4 is consumed specifically by the classical complement pathway—the pathway activated by antibody-antigen immune complexes. Low C4 is even more sensitive than low C3 for detecting active lupus. Some people also have genetic partial C4 deficiency, which itself predisposes to developing lupus.
What is Complement C4?
C4 is part of the classical and lectin complement pathways. Normal: 10-40 mg/dL. More sensitive than C3 for classical pathway activation (lupus). Genetic C4 deficiency (C4A or C4B null alleles) is relatively common and predisposes to SLE.
↑ What High Complement C4 Means
Acute phase reactant. Mild elevation is common with inflammation.
Common symptoms:
Nonspecific inflammation
↓ What Low Complement C4 Means
Classical pathway activation (immune complex disease, especially lupus) or genetic deficiency. C4 drops earlier than C3 during lupus flares and is a more sensitive flare marker.
Common symptoms:
If lupus flare: fatigue, joint pain, rash, fever · If HAE: recurrent episodes of angioedema (face, limbs, larynx, GI tract) · If genetic: often no symptoms
Why It Matters
When normal:
More sensitive than C3 for lupus activity
C4 drops before C3 in early lupus flare
Persistent low C4 suggests genetic deficiency
Monitors response to lupus treatment
Risks if abnormal:
Low: active lupus, angioedema (C1-inhibitor deficiency), immune complex disease
Genetic C4 deficiency: predisposes to SLE development
Very low + active symptoms = aggressive disease
What Can Cause Abnormal Levels?
Active SLE
45% likelyClassical pathway consumption. C4 drops earlier and more sensitively than C3.
Genetic C4 Deficiency
25% likelyC4A null alleles are common. Persistent isolated low C4 may be genetic, not pathological.
Hereditary Angioedema
C1-inhibitor deficiency consumes C4. Screen for HAE if recurrent angioedema with low C4.
Cryoglobulinemia
Especially HCV-related. Low C4 is characteristic.
Immune Complex Disease
Same conditions as C3 consumption.
What You Can Do
If persistently low C4 without disease activity: may be genetic C4 deficiency
Impact: Common finding—doesn't always mean active disease \u00B7 Timeline: Confirm with repeat
Low C4 + recurrent angioedema: check C1-inhibitor level and function
Impact: Screens for hereditary angioedema \u00B7 Timeline: One-time
If lifestyle changes aren't enough:
In lupus: trending C4 over time is more useful than a single value
Impact: Dropping C4 predicts flare \u00B7 Timeline: q3-6 months
Recommended retest: q3-6 months in SLE; confirm genetic deficiency with repeat testing
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