Evaluation of six tests for circulating IgG complexes with special reference to IgM rheumatoid factors: analysis of systemic lupus erythematosus and rheumatoid arthritis series.

Six tests for circulating immune complexes (CIC) developed in four laboratories and representing four main principles [affinity of human platelets, Clq, of RF for aggregated IgG, and of conglutinin (Kg) for complex-bound C3] were evaluated on series of SLE and definite RA. All tests detected human model complexes in the presence of NHS and discriminated the patient series from the blood donor series, most powerful being the PIPA (platelet test). The high correlation between the RF-binding inhibition tests (RFbI) and the RF-latex test suggested interference due to intrinsic RFs. This received further support from experimental analyses in which RA sera mixed with varying doses of heat-aggregated IgG were assayed by the pRFbI test.