Complement (C3 & C4)

The complement system is a protein cascade of 30 enzymes and activator and control proteins, two-thirds of which circulate in the plasma while the remainder are present on cell and tissue surfaces. Complement levels may increase following trauma or inflammation as part of the acute phase response. Complement consumption can also take place in certain disease states, and complement concentration can be useful in diagnosis and monitoring progress. Low concentrations indicate increased consumption or decreased synthesis.

C3 and C4 levels are useful in the management of patients with renal disease, inflammatory joint disease, and multi-system disorders with evidence of vasculitis. A low complement concentration would suggest an immunological basis for the symptoms. C3 and C4 concentrations are often low inactive SLE and cryoglobulinaemia.

Persistently low C3 in glomerulonephritis suggests the presence of C3 nephritic factor and in haemolytic uraemic syndrome Factor H deficiency.
C4 is decreased in hereditary and acquired C1 inhibitor deficiency.
Complement studies are indicated in individuals with recurrent pyogenic septicaemia/meningitis.