C3 and C4 levels would be expected to be raised in systemic inflammatory response, usually up to twice their resting levels. In this setting a normal or low level may reflect utilisation, such as in an overwhelming infection. C4 levels will be decreased if utilisation by the classical pathway (e.g. in immune complex disease such as SLE and vasculitis associated with rheumatoid arthritis) exceeds production. C4 levels are low in patients with C1 inhibitor deficiency, and may be absent during an attack - normal levels effectively exclude the diagnosis. Low C3 reflects activation of either the classical or alternate pathways. Low C3 with a normal C4 occurs with isolated alternative pathway activation (e.g. type II mesangiocapillary GN, due to production of an autoantibody (C3 nephritic factor) that activates this pathway directly).
Complement Components -C3 and C4
Ambient (8 - 24 degrees Celsius)
Elevated C3 and C4 concentrations occur in inflammatory disorders.
Decreased C3 concentrations occur in SLE, post-infectious glomerulonephritis, glomerulonephritis associated with C3 nephritic factor, rheumatoid vasculitis, hereditary deficiency and severe liver disease.
Decreased C4 concentrations occur in active SLE, hereditary angiooedema and hereditary deficiency states.
C3: 0.9 – 1.8 g/L
C4: 0.10 – 0.4 g/L
Immunoassay by Siemens BNII nephelometer.
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