Modified anti-C3 immune complex assay which avoids interference by anti-F(ab')2 antibodies

The present authors and Olds et al. reported that the anti-F(ab')2 antibodies (Abs) in serum interfere with the solid phase (SP) anti-C3 immune complex assay. The anti-F(ab')2 Abs in human sera bind solid phase F(ab')2 anti-C3 of rabbit or goat, and were measured erroneously as C3 bearing circulating immune complexes (CIC). Gel filtration analysis of SP anti-C3 assay revealed that C3 bearing CIC is detected only in heavy fractions and 7S CIC-like activity is not CIC but anti-F(ab')2 activity. As the molecular weight of such CIC is heavy enough to be precipitated by 5% polyethylene glycol (PEG) and IgG anti-F(ab')2 Abs and free C3 are not included in 5% PEG precipitates, 5% PEG precipitates of the test sera were used for SP anti-C3 (Modified SP anti-C3). CIC measured by modified SP anti-C3 were positive in 14/16 at active stage of SLE and positive only in 2/16 at inactive stage. CIC by this test were also correlated well to serum complement activity, and were thought to be clinically reliable and useful.