FRI0588 DIAGNOSTIC PERFORMANCE OF THE RHEUMATOID FACTOR ISOTYPES IN RHEUMATIC DISEASES

Background:Rheumatoid Factor (RF) is used for the classification of Rheumatoid arthritis (RA) in patients whose main complaint is hand arthritis but is well known that other conditions have positive RF. Differences in analytical performance between assays were found in previous studies, there was a poor numerical agreement, and using the cut-off proposed by the manufacturer, variation in sensitivity and specificity can be seen.2Objectives:To determine the cut-off value for RF with the best diagnostic performance for RA in patients with hand arthritis compared with their main differential diagnoses.Methods:Descriptive, cross-sectional, diagnostic test study. Sampling size was calculated 2:1, with the area under the ROC curve (AUC): 0.708, power 80% and significance 0.05. Patients with RA, Systemic Lupus Erythematosus (SLE), Osteoarthritis (OA), fibromyalgia (FBM), Polyarticular Gout (PG), Psoriatic Arthritis(Psa), Sjögren Syndrome (SS), Arthralgia suspicious for progression to RA (ASP) and healthy subjects, previously classified with the current criteria by certified rheumatologist were included. EUROINMMUN ® assays for RF isotypes IgA, IgM, IgG were used. ROC curves and Youden index for determining the cut-off with the best sensitivity and specificity for each RF isotype comparing RA with other diseases were used. Statistical analyses were performed with SPSS V.24.Results:Three hundred and thirty-two serum samples from patients were included, in the other rheumatic diseases different from RA, we found positive RF IgM in 141 (45.5%), 82 (26.5%) IgA and 25 (8.1%) IgG. For each clinical scenario the cut-off with the best sensitivity and specificity was different from the manufacturer proposed, and isotypes with the best diagnostic performance was not the same for each disease, Table 1. We present the ROC curve for RF isotypes in Figure 1.Conclusion:In order to improve the diagnosis of Rheumatoid Arthritis, different cut-off points of RF and isotypes should be used.References:[1] Van Hoovels L, Jacobs J, Vander Cruyssen B, Van Den Bremt S, Verschueren P, Bossuyt X. Performance characteristics of rheumatoid factor and anti-cyclic citrullinated peptide antibody assays may impact ACR/EULAR classification of rheumatoid arthritis. Ann Rheum Dis. 2018;77(5):667-677. doi:10.1136/annrheumdis-2017-212365[2] Van Boekel MAM, Vossenaar ER, Van Den Hoogen FHJ, Van Venrooij WJ. Autoantibody systems in rheumatoid arthritis: Specificity, sensitivity and diagnostic value. Arthritis Res. 2002;4(2):87-93. doi:10.1186/ar395.Disclosure of Interests:None declared