Anti-DNA antibodies: the choice of assays for routine diagnostic work

Sixty-six sera from 23 patients with systemic lupus erythematosus (SLE), 26 sera from patients with rheumatoid arthritis (RA), and 22 sera from normal healthy subjects were tested for the presence of antibodies against native (ds) DNA by the Crithidia luciliae immunofluorescence test and by the Farr assay, and for the presence of antibodies against denaturated (ss) DNA by the enzyme-linked immunosorbent assay (ELISA). Anti-dsDNA antibodies were detected in 57% of the SLE patients by the Crithidia test and in 65% by the Farr assay. Two of the RA sera were positive in the Crithidia test, whereas all were Farr negative. Anti-ssDNA antibodies of IgG class could be detected in 74% of the SLE patients and in none of the RA sera, while anti-ssDNA antibodies of IgM class were found in 26% of the SLE patients and in one RA serum. There was a good correlation between the results of the Farr assay and the IgG-anti-ssDNA ELISA but no agreement was found between the results of the Farr assay and the Crithidia test. We also measured the amount of C-reactive protein (CRP) in the sera but no correlation was seen between the levels of CRP and anti-DNA antibodies. We conclude that the demonstration of anti-ssDNA antibodies of IgG class is a good screening method in the diagnosis of SLE, and that antibodies against native DNA should be determined, preferably both by the Crithidia test and the Farr assay to confirm the diagnosis and in the follow-up of the patients.