RT Journal Article SR Electronic T1 Multicentre study to improve clinical interpretation of rheumatoid factor and anti-citrullinated protein/peptide antibodies test results JF RMD Open JO RMD Open FD EULAR SP e002099 DO 10.1136/rmdopen-2021-002099 VO 8 IS 1 A1 Lieve Van Hoovels A1 Bert Vander Cruyssen A1 Daniela Sieghart A1 Carolien Bonroy A1 Eszter Nagy A1 Rille Pullerits A1 Saša Čučnik A1 Charlotte Dahle A1 Ingmar Heijnen A1 Luca Bernasconi A1 Farid Benkhadra A1 Laura Bogaert A1 Stefanie Van Den Bremt A1 Ann Van Liedekerke A1 Geert Vanheule A1 Johan Robbrecht A1 Lucy Studholme A1 Claudine Wirth A1 Rüdiger Müller A1 Diego Kyburz A1 Christopher Sjöwall A1 Alf Kastbom A1 Rok Ješe A1 Boja Jovancevic A1 Emese Kiss A1 Peggy Jacques A1 Daniel Aletaha A1 Guenter Steiner A1 Patrick Verschueren A1 Xavier Bossuyt YR 2022 UL http://rmdopen.bmj.com/content/8/1/e002099.abstract AB Background Rheumatoid factor (RF) and anti-citrullinated protein/peptide antibodies (ACPA) are important biomarkers for diagnosis of rheumatoid arthritis (RA). However, there is poor harmonisation of RF and ACPA assays. The aim of this study was to refine RF and ACPA interpretation across commercial assays.Materials and methods Six total RF isotype-non-specific assays, 3 RF IgM isotype-specific assays and 9 ACPA immunoglobulin G assays of 13 different companies were evaluated using 398 diagnostic samples from patients with RA and 1073 disease controls.Results Using cut-offs proposed by the manufacturer, there was a large variability in diagnostic sensitivity and specificity between assays. Thresholds of antibody levels were determined based on predefined specificities and used to define test result intervals. Test result interval-specific likelihood ratios (LRs) were concordant across the different RF and ACPA assays. For all assays, the LR for RA increased with increasing antibody level. Higher LRs were found for ACPA than for RF. ACPA levels associated with LRs >80 were found in a substantial fraction (>22%) of patients with RA.Conclusion Defining thresholds for antibody levels and assigning test result interval-specific LRs allows alignment of clinical interpretation for all RF and ACPA assays.All data relevant to the study are included in the article or uploaded as supplementary information.