Complement Component 4
Inflammation marker
Complement C4
Complement Component 4
A protein of the classical complement pathway. Like C3 it is consumed in active immune-complex disease (especially lupus) and can rise modestly as an acute-phase reactant, but C4 often falls earlier and further than C3 in lupus flares.
PED Notes
Complement C4 is not affected by AAS, GH, or peptides. It is used alongside C3 in autoimmune assessment: a low C4, particularly with a low C3 and positive anti-dsDNA, indicates active systemic lupus, which can be mistaken for the fatigue and joint pain of hard training. C4 is often the more sensitive of the two in lupus, sometimes dropping before C3. It can rise mildly as an acute-phase protein, so a high value is non-specific. The clinically important direction is LOW (consumption). The bodybuilder range equals the standard range.
When high
When elevated (>0.4 g/L):
- A raised C4 is a non-specific acute-phase response to inflammation, infection, or injury.
- What to do: usually no action for the C4 itself. Confirm a rested draw and read alongside CRP and ESR; investigate a chronic inflammatory source only if other markers agree.
When low
When low (<0.1 g/L) -- the clinically important direction:
- A low C4 indicates classical-pathway complement consumption, most notably in active systemic lupus and lupus nephritis; it can also reflect inherited C4 deficiency (a recognised lupus risk factor) or, rarely, hereditary angioedema.
- What to do:
- Seek medical review, especially with a low C3, positive ANA or anti-dsDNA, rash, arthritis, or kidney involvement.
- Trend C3 and C4 together; a combined fall supports an active lupus flare.
- Pair with urinalysis and kidney function to screen for lupus nephritis.
History Chart
Reading History
Frequently Asked Questions
Reference Ranges
Standard Range
VitalMetrics Range