Complement C₃

Serum (preferred) or plasma

1 mL (adult), 0.5 mL (pediatric)

Red-top tube, gel-barrier tube, or green-top (heparin) tube.

Quantitation of C₃ is used to detect individuals with inborn deficiency of this factor or those with immunologic disease in whom complement is consumed at an increased rate. These include lupus erythematosus, chronic active hepatitis, certain chronic infections, poststreptococcal and membranoproliferative glomerulonephritis, and others.

Detects both biologically active and inactive C₃

Immunologic

See table.

C₃ comprises about 70% of the total protein in the complement system and is central to activation of both the classical and alternate pathways. Increased levels are found in numerous inflammatory states as an acute phase response. CH₅₀ (total complement hemolytic activity), C₃ and/or C₄ may be decreased in cases of systemic lupus erythematosus, especially in cases with lupus nephritis, acute and chronic hypocomplementemic nephritis, subacute bacterial endocarditis, DIC, and partial lipodystrophy (with associated nephritis-like activity in serum.) In cases of disseminated intravascular coagulation, plasmin attacks C₃ directly, and C₃ levels have been found low in the hemolytic uremic syndrome form of disseminated intravascular coagulation (DIC). Cases of hereditary C₃ deficiency, while rare, have been reported and are characterized clinically by recurrent infections (eg, pneumonia, meningitis, paronychia, impetigo). Pathogenic bacteria causing infections in these cases have included both gram-positive and gram-negative organisms. C₃ levels have also been found deficient in cases of uremia, chronic liver diseases, anorexia nervosa, and celiac disease.