269 - Validation of a new ELISA method for the detection of RNAP III antibodies in systemic sclerosis
Autor(s): R. Tozzoli, G. Kodermaz, G. Morozzi, V. Codullo, S. Platzgummer, E. Tonutti, M. Tampoia, D. Bassetti, N. Bizzaro, A. Ruffatti, G. Valesini, A. Tincani, G. De Leopardi, O. De Pità
Issue: RIMeL - IJLaM, Vol. 3, N. 4, 2007 (MAF Servizi srl ed.)
Page(s): 269-74
Background. Systemic sclerosis (SSc) is an autoimmune disease, characterized by the presence of specific autoantibodies, such as anti-topoisomerase I (anti-topo I) and anticentromere antibodies (ACA). However, about 50% of SSc patients have only anti-nucleolar antibodies (such anti-RNA-polymerase (anti-RNAP), anti- PM/Scl, anti-Th/To, antifibrillarin) which can be identified only by means of in-house methods. The aims of the present study were: a) evaluate the diagnostic accuracy of a new ELISA method for the detection of anti-RNAP in a group of SSc patients; b) evaluate the diagnostic sensitivity of anti-RNAP in patients who are negative for anti-topo I and ACA; c) estimate the concordance with the reference method (radio-immunoprecipitation assay, RIPA); and d) assess the analytical association with antinuclear fluorescence patterns and with the clinical subsets of the disease. Methods. Sera from 207 patients with SSc and from 188 controls were assayed using a new ELISA method with a recombinant fragment containing the immunodominant epitope of RNA polymerase III. The results were compared with those obtained by a homemade RIPA. Results. Using the ROC curve analysis, two cut-off values were defined: 12 U/mL provided the best diagnostic sensitivity (sensitivity: 27.2%, specificity: 95%) and 35 U/mL the best nosographic specificity (sensitivity: 27.2%, specificity: 99%). None of SSc patients who were positive to anti-topo I or to ACA were also positive for anti-RNAP. The analytical concordance with the reference method was calculated in a group of 91 patients, and was 100%. High levels of anti-RNAP were associated with anti-nuclear speckled or nucleolar fluorescence patterns and with the diffuse cutaneous form of the disease. Conclusions. The new anti-RNAP III ELISA method is analytically accurate and clinically specific. Testing for this type of autoantibody is now routinely available, and may help diagnosing about 25% of SSc patients who are negative to other SSc-related autoantibodies, using a positivity threshold of 35 U/mL.