[Reassessment of measurement of anti double-stranded DNA antibodies by Farr's assay using double-stranded DNA from E. coli and application of DNA binding Activities in high salt solution].

Farr's assay using double-stranded (ds) DNA from E. coli is a most sensitive and specific method for the detection of anti ds-DNA antibodies in patients with systemic lupus erythematosus (SLE). Because of the lack of sufficient DNA antigens, however, final antibody titers were hardly determined when the sera contained antibodies titered more than 100U/ml (or more than 60 per cent by DNA binding activities). In such sera DNA binding activities were measured by using ds-DNA tracer adjusted final concentration of NaCl to 125 mM. Higher binding activities measured by high-salt tracer are obtained significantly in SLE patients groups with nephrotic syndrome, proteinuria, cast, renal failure, diffuse proliferative nephritis, low serum complement levels, anemia and/or low IgG/IgA levels compared with the patients who lacked these clinical findings. In contrast the patients with digital rash or cramp showed significantly lower high-salt binding activities. The patients with pleuropericarditis tended to have lower bindings. The non-lupus patients including MCTD also had lower levels. These clinical characteristics could not be evaluated by standard Farr's assay. High-salt bindings suggest the presence of high avidity antibodies and also partly may mean the high levels of low avidity antibodies. The application of high-salt binding activities, thus, is a useful tool for the evaluation of clinical characteristics of SLE patients who had high levels of anti ds-DNA antibodies by standard Farr's assay.