Test Background Lowered C3 or C4 levels may be due to excessive consumption of the complement cascade, as seen in immune complex-mediated disease, such as SLE with associated renal disease, or cryoglobulinaemic nephritis. May indicate disease flare or aid prognosis. Also, components can be low due to genetic/acquired defects. Commonest cause of low C3 and C4 is sample degradation. Raised levels may indicate an acute phase response.
Clinical Indications Diagnosis of glomerulonephritis Diagnosis of HAE or cryoglobulinaemia Diagnosis or exclusion: acute phase increases vs. underlying paraprotein pathology (serum electrophoresis reflex testing) Some utility in monitoring autoimmune conditions, such as SLE Single point determinations of C3 and C4 are of limited value and serial measurements are recommended.