Complement fixation by anti-dsDNA antibodies in SLE: measurement by radioimmunoassay and relationship with disease activity.

We have developed a sensitive, solid phase radioimmunoassay (RIA) to quantify the amount of complement (C') fixation by anti-double-stranded DNA (anti-dsDNA) antibodies and have studied sera from 48 patients with systemic lupus erythematosus (SLE). 46% of the patients were positive in this assay. There was no correlation with serum C' levels, and only weak correlation with anti-dsDNA activity as measured by a solid phase RIA. An association was shown between positive values for C' fixation by anti-dsDNA antibodies and the presence of active lupus (p less than 0.01); there was a similar association with the presence of active renal disease in those patients with raised DNA binding (p less than 0.01). Our results suggest that quantitative measurement of C' fixation by anti-dsDNA antibodies may provide information about the pathogenicity of such antibodies in patients with SLE and be a better guide to potential end organ damage than the conventional measurement of DNA binding.