Clinical significance of anti-nDNA antibodies in ANA-positive systemic lupus erythematosus: comparison of the Farr radioimmunoassay and the Crithidia luciliae immunofluorescent technique.

To evaluate the clinical applicability of anti-native DNA antibodies (anti-nDNA) in systemic lupus erythematosus (SLE), two conventional methods--the Farr radioimmunoassay (Farr assay) and the Crithidia luciliae immunofluorescent technique (CL-IF assay)--were compared in 180 sera of antinuclear antibody (ANA)-positive systemic lupus erythematosus (85 active and 95 inactive), 31 sera of ANA-positive non-SLE rheumatic diseases and 40 sera from healthy subjects. The results further appraised the clinical significance of anti-nDNA antibodies in the diagnosis of systemic lupus erythematosus, especially those patients with nephritis. The CL-IF assay was found less sensitive in the detection of anti-nDNA than the Farr assay. As a tool for differentiating SLE from non-SLE rheumatic diseases, the CL-IF assay is not as sensitive as the Farr assay (sensitivity of CL-IF and Farr assay = 38.3% versus 60.0%). But the CL-IF test is much more convenient and efficient in the detection of active stage SLE and lupus nephritis than the Farr assay. The efficiency of anti-nDNA tests by the CL-IF and Farr assays for diagnosing active SLE were 85.8% versus 79.6%, for lupus nephritis were 76.1% versus 64.4% and finally for both conditions were 81.7% versus 62.2%. As an index of disease activity during the course of treatment, titers of anti-nDNA antibodies by Farr assay proved much more reliable than those by Crithidia luciliae assay. For the detection of anti-nDNA antibodies, simultaneous utilization of these two methods for fresh cases and using the Farr assay alone during the course of treatment are recommended.