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Antibodies targeted against double-stranded DNA (dsDNA) were first described in 1957 (1, 2). Detection of anti-ds-DNA antibody (ds-DNA-Ab) is an integral part of the definition of systemic lupus erythematosus (SLE) as proposed by the American College of Rheumatology (ACR) criteria (3). Two of 11 criteria set by the ACR are related to anti-nuclear antibody detection (anti-nuclear antibodies; anti-ds-DNA or anti-sm or anti-phospholipid antibodies). Because the diagnosis of SLE is established when 4 of the 11 criteria are fulfilled, the correct identification of antibodies is extremely important. Because dsDNA-Ab may occur in a wide spectrum of diseases and even in healthy volunteers, diagnostic tests must be highly specific. The Farr RIA is assumed to meet this need best (4, 5). To improve the diagnostic accuracy of dsDNA-Ab tests, new tests have been developed, including ELISAs with different preparations of ds-DNA antigens as well as an automated enzyme fluoroimmunoassay using plasmid DNA as antigen. However, clinical data to assess the diagnostic accuracy of these recently released test systems are limited at present. To clarify the clinical usefulness of different dsDNA-Ab tests in the diagnosis of SLE, we measured ds-DNA-Ab in 152 Caucasian patients for whom analysis of anti-nuclear antibodies was requested between November 1999 and September 2000 in the Laboratory of Immunology at the University of Bonn. To take into account differences in diagnostic standards, the patient records were reevaluated by the study personnel, who used a unified protocol to verify the presence of diagnostic criteria according to the current revision of the ACR (3). In 12 of 152 patients, a definite decision on the presence of SLE-associated manifestations could not be made because the records were incomplete. These 12 patients were excluded from further analysis. All study procedures were performed in accordance with the current revision of the Helsinki Declaration of 1975, and the study design was based on the 2000 guidelines for evaluation of diagnostic accuracy as suggested by Bruns et al. (6).