Detects both biologically active and inactive C3. Complement proteins are acute phase reactants and have short half-lives.
Serum level is a balance of synthesis and catabolism.
Serial measurements are more useful than single values. These include lupus erythematosus, chronic active hepatitis, certain chronic infections, poststreptococcal and membranoproliferative glomerulonephritis, and others. Quantitation of C4 is used to detect individuals with inborn deficiency of this factor or those with immunologic disease in whom hypercatabolism of complement causes reduced levels. These diseases include lupus erythematosus, serum sickness, certain glomerulonephritides, chronic active hepatitis, and others. Increased levels are found in numerous inflammatory states as an acute phase response.