Scientific Abstracts Friday, 05 June 2020 899 FRI0588 DIAGNOSTIC PERFORMANCE OF THE RHEUMATOID FACTOR ISOTYPES IN RHEUMATIC DISEASES R. Pineda-Sic 1 , D. Vega-Morales 1 , L. Santoyo-Fexas 1 , M. M. Castañeda- Martínez 1 , I. D. J. Hernandez-Galarza 1 , M. A. Villarreal-Alarcón 1 , J. C. Riegatorres 1 , L. Pérez Barbosa 1 , D. E. Flores Alvarado 1 , M. Eguia Bernal 1 , G. Serna-Peña 1 , D. Á. Galarza-Delgado 1 . 1 Universidad Autónoma de Nuevo Léon, Monterrey, Mexico Background: Rheumatoid Factor (RF) is used for the classifcation of Rheuma- toid arthritis (RA) in patients whose main complaint is hand arthritis but is well known that other conditions have positive RF. Differences in analytical perfor- mance between assays were found in previous studies, there was a poor numer- ical agreement, and using the cut-off proposed by the manufacturer, variation in sensitivity and specifcity can be seen. 2 Objectives: To determine the cut-off value for RF with the best diagnostic perfor- mance 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 signifcance 0.05. Patients with RA, Systemic Lupus Erythematosus (SLE), Osteoarthritis (OA), fbromyalgia (FBM), Polyarticular Gout (PG), Psoriatic Arthritis(Psa), Sjögren Syndrome (SS), Arthralgia suspicious for progression to RA (ASP) and healthy subjects, previously classifed with the current criteria by certifed rheumatologist were included. EUROINMMUN ® assays for RF iso- types IgA, IgM, IgG were used. ROC curves and Youden index for determining the cut-off with the best sensitivity and specifcity 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 specifcity was different from the manufac- turer 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 classifcation 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: Specifcity, sensitivity and diag- nostic value. Arthritis Res. 2002;4(2):87-93. doi:10.1186/ar395. Disclosure of Interests: None declared DOI: 10.1136/annrheumdis-2020-eular.4039 FRI0589 CLINICAL ACCURACY OF ANTI- PHOSPHATIDYLSERINE/PROTHROMBIN AND ANTI- BETA2GLYCOPROTEIN I DOMAIN I FOR PREDICTING CLINICAL MANIFESTATIONS OF ANTIPHOSPHOLIPID SYNDROME M. Radin* 1 , I. Cecchi 1 , S. G. Foddai 1 , E. Rubini 1 , A. Barinotti 1 , S. Sciascia 1 , D. Roccatello 1 . 1 University of Turin, Turin, Italy Background: Antiphospholipid antibody (aPL) specifcities are increasing, how- ever the best accuracy for predicting the clinical manifestations of the diseases is still under debate. Objectives: To evaluate the clinical accuracy of aPL specifcities both individually and/or in combination, to identify a panel of tests that may provide the best accu- racy for predicting clinical manifestations of antiphospholipid syndrome (APS). Methods: We chart-reviewed patients that presented at San Giovanni Bosco Hospital in the last 5 years who tested persistently positive for at least one aPL[1]. Inclusion criteria comprehended: a)Primary APS(PAPS); b) Secondary APS(SAPS), c)aPL positive asymptomatic and d)Patients with clinical manifes- tations of APS and either inconsistent previous LA positivity and/or low-medium titers aPL(suspect APS). Results: A total of 122 patients were included in the study 38 PAPS, 31 SAPS, 23 aPL positive asymptomatic and 30 low titers APS patients). Table1 resumes the main clinical and laboratory characteristics of the patients included in the study. All patients were tested for fve aPL: criteria aPL, anti-phosphatidylserine/prothrom- bin(aPS/PT) and anti-Beta2Glycoprotein I Domain I(β2GPI-D1). Among the single aPL positivity, β2GPI-D1 showed the best accuracy in predict- ing the clinical manifestations of APS (AUC 0.639,CI95% 0.539–0.74,p=0.011). Similarly, when investigating the possible combinations of aPL panels, the positiv- ity for aPS/PT IgG/IgM and/or β2GPI-D1 showed the best accuracy in predicting the clinical manifestations of APS (AUC 0.698, CI95%0.604 –0.793, p<0.001) while triple criteria aPL IgG positivity had the best accuracy for predicting recur- rences (AUC 0.720, CI95% 0.569–0.871, p=0.012). Results of the ROC curves are illustrated in Figure 1. Table 1. RF diagnostic performance in rheumatic diseases Group n Isotype** Cut-off UR/ml Signifcance AUC (95% CI) Sensitivity % Specifcity % Youden Index Sensitivity % Manufacturer cut-off + Specifcity % Manufacturer cut-off + RA 22 IgM 135.3 0.06 0.722(0.604 to 0.839) 60% 85.2 0.45 68.2 55.5 Psa 44 IgA 47.2 0.074 0.698(0.553 to 0.842) 54.5 81.8 0.35 54.5 77.3 ASP 44 IgA 39.5 0.080 0.668(0.511 to 0.826) 54.5 88.6 0.43 54.9 79.5 SS 44 IgM 180.6 0.088 0.535(0.088 to 0.708) 54.5 74.4 0.28 68.2 9.3 Healthy 44 IgM 16.3 0.046 0.896(0.806 to 0.986) 77.3 88.9 0.66 68.2 97.8 SLE 41 IgA 42.6 0.073 0.701(0.557 to 0.845) 54.5 85.3 0.39 54.5 73.5 FBM 35 IgM 68.6 0.071 0.752(0.612 to 0.892) 63.6 82.8 0.46 68.2 51.7 OA 38 IgM 48.0 0.053 0.873(0.770 to 0.976) 63.6 96 0.59 68.2 88 PG 20 IgM 117.0 0.076 0.758(0.609 to 0.908) 59.1 89.5 0.49 68.2 63.2 Total 332 ** The isotype with the best AUC for each clinical scenario. + Manufacturer cut-off value: 20UR/ml on February 23, 2022 by guest. 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