Statistical analysis of five immune complex screening assays: patterns of detection in patients with rheumatoid arthritis, systemic lupus erythematosus, infectious endocarditis, and diabetes mellitus.

A comparative study of four nonspecific screening techniques (direct nephelometry, PEG-C4, PEG-IgG, and radiolabeled Clq binding) for immune complexes (IC) and of a technique specific for the detection of insulin-anti-insulin IC was undertaken in four groups of patients with diagnosis of infectious endocarditis, systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), and diabetes. The highest frequency of positive results was given by the PEG-IgG test in RA, the Clq-binding test in SLE, the insulin-anti-insulin IC screening test in diabetes, and the PEG-IgG and Clq-binding tests in infectious endocarditis. Of the four nonspecific tests, the PEG-C4 assay appeared to be the least discriminative, since it failed to show significant differences between any group of patients and the group of controls. Direct nephelometry, PEG-IgG, and radiolabeled Clq binding gave consistently higher results in RA than in other diseases, and in this disease the rates of agreement between these tests were highly significant. Significant agreements between the rates of positivity of Clq binding and PEG-IgG tests were seen in all groups of patients studied. Spearman's analysis of rank showed the best correlations among tests based on similar principles (ie, PEG precipitation), and also a strong correlation between Clq binding and the PEG-IgG test in RA. The PEG-IgG test appears to be a reliable IC screening test for general use with the advantage of not involving radioisotopes. In regard to antigen-specific tests, although their specificity and sensitivity may be high, their results may show no correlation with nonspecific screening tests nor with the presence or absence of clinical or laboratory abnormalities suggestive of IC deposition, as exemplified by the insulin-anti-insulin IC screening test in diabetic patients.