Immune complex assays in rheumatic diseases.

A wide variety of antigen-nonspecific immune complex assays have been developed in recent years for the detection and quantitation of immune complexes in pathologic fluids. These assays detect complexed antibody regardless of the antigen involved. Almost all of these assays use biologic reagents that may react with substances other than complexed antibody. In addition, the assays do not differentiate nonspecifically aggregated antibody from antigen-complexed antibody. Hence, these assays are not absolute tests for immune complexes. On the basis of studies using these assays, "immune complexes" have been detected in a large number of rheumatic diseases. While these findings have been of considerable investigative interest, thus far they have been of little practical clinical utility. The detection of immune complexes has not been shown to be essential in any clinical conditions but may be helpful in monitoring disease activity in systemic lupus erythematosus (SLE) and may provide useful diagnostic information in two rare syndromes, Lyme arthritis and SLE-related syndrome.