Complement components C3 and C4

Keywords: C3, C4


5mL blood in plain tube.

The specimen should be placed once and transported immediately to the laboratory.



Functional assay if primary deficiency is suspected.

Reference Interval:

C3: 0.9-1.8 g/L

C4: 0.16-0.50 g/L.


Monitoring activity of immune complex mediated disease eg, SLE, Cryoglobulinaemia.

Detection of primary deficiency of C3 or C4 (rare).

Assessment of angioedema, see also C1 esterase inhibitor.


Reduced C4 with or without reduced C3: activation by classical pathway (eg, SLE, immune complex disease / Cryoglobulinemia).

Reduced C3 with normal C4: activation by the alternative pathway (eg, sepsis, endotoxin).

Raised C3 and/or C4: acute phase response.

Reduced C4 normal C3 colon C1 inhibitor deficiency


Giglas P. In: Rose NR et al eds. Manual of Clinical Laboratory Immunology. 5th ed. ASM Press 1997.

Wen L et al. Journal of Allergy and Clinical Immunology. 2004; 113: 585-93.