Friday, March 20, 2015

C3

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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