The
classic and alternative complement pathways converge at the C3 step in the
complement cascade. Low levels indicate activation by one or both pathways.
Most diseases with immune complexes will show decreased C3 levels. Test
as usually performed is an immunoassay (by radial immunodiffusion or nephelometry).
Normal
Range: 64-166 mg/dL
increased
in: Many inflammatory conditions as an acute phase reactant, active phase
of rheumatic diseases (rheumatoid arthritis, SLE, etc), acute viral hepatitis,
myocardial infarction, cancer, diabetes, pregnancy, sarcoidosis, amyloidosis,
thyroiditis.
Decreased by: Decreased
synthesis (protein malnutrition, congenital deficiency, severe liver disease),
or increased catabolism (immune complex disease, membranoproliferative
glomerulonephritis [75%], SLE, Sjogren's, rheumatoid arthritis, disseminated
intravascular coagulation, paroxysmal nocturnal hemoglobinuria, autoimmune
hemolytic anemia, gram-negative bacteremia) and increased loss (burns,
gastroenteropathies).
Additional: Complement
C3 levels may be useful in following the activity of immune complex diseases.
The best test to detect inherited deficiencies is CH50. Levels can confirm
specific C3 defect.
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