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Systemic lupus erythematosus (SLE) is a systemic autoimmune disease belonging to the group of connective tissue diseases. The classification criteria of the European League Against Rheumatism (EULAR) and the American College of Rheumatology (ACR) revised in 2019 contain 7 clinical and 3 immunological categories with the individual parameters weighted using a point system. A case is classified as SLE if the entry criterion of a positive ANA test is fulfilled and the total score is at least 10.
Antibodies against double-stranded DNA (dsDNA) are the main focus of serological SLE diagnostics. These antibodies can be found in 60% to 90% of patients, depending on the activity of the disease. In rare cases, anti-dsDNA antibodies are also found in patients with other autoimmune diseases (e.g. autoimmune hepatitis) or infections and in clinically healthy persons. 85% of people in the latter group develop SLE within 5 years of initial detection of anti-dsDNA. However, SLE cannot be excluded if anti-dsDNA antibodies are not detected.
Antibodies against nucleosomes are also an exclusive marker of SLE, provided that they are determined using a test system with a target antigen that is free of histone H1, Scl-70 and other non-histone proteins. Anti-Sm antibodies are also highly specific markers but occur a lot less frequently. Anti-Sm and anti-dsDNA antibodies each have a high weighting of 6 points in the classification criteria.
Various test methods are available for the routine detection of antibodies against dsDNA: enzyme immunotests (ELISA, EUROASSAY, EUROLINE), Farr RIA and the Crithidia luciliae immunofluorescence test (CLIFT). The various test systems differ, sometimes greatly, regarding their sensitivity and specificity. The conventional CLIFT, for example, shows a particularly high disease specificity, while the IIFT Crithidia luciliae sensitive was developed with a focus on high sensitivity.
Thanks to the use of highly purified nucleosomes as linking substance, the performance data of the Anti-dsDNA-NcX ELISA are significantly higher than those of the conventional Anti-dsDNA ELISA. Because of their strong adhesive ability, even a very low concentration of these nucleosomes is suited to coupling isolated dsDNA to the surface of a microplate well. False positive reactions due to conventional linking substances such as poly-L lysine and protamine sulphate are avoided.
In a clinical comparative study of 564 patients with rheumatic diseases (of these 207 with SLE), the Anti-dsDNA-NcX ELISA yielded an 8% higher sensitivity than the Anti-dsDNA RIA (Biesen et al., Arthritis Res Ther (2011) 13:R26). Nevertheless, different test methods identify different SLE subgroups, so different test systems should be combined to increase the serological detection rate.