OBJECTIVE: Ankylosing Spondylitis Disease Activity Score based on C-reactive protein (ASDAS-CRP) is recommended over ASDAS based on erythrocyte sedimentation rate (ASDAS-ESR) to assess disease activity in axial spondyloarthritis (axSpA). Although ASDAS-CRP and ASDAS-ESR are not interchangeable, the same disease activity cut-offs are used for both. We aimed to estimate optimal ASDAS-ESR values corresponding to the established ASDAS-CRP cut-offs (1.3, 2.1, and 3.5) and investigate the potential improvement of level of agreement between ASDAS-ESR and ASDAS-CRP disease activity states when applying these estimated cut-offs. METHODS: We used data from patients with axSpA from 9 European registries initiating a tumor necrosis factor inhibitor. ASDAS-ESR cut-offs were estimated using the Youden index. The level of agreement between ASDAS-ESR and ASDAS-CRP disease activity states was compared against each other. RESULTS: In 3664 patients, mean ASDAS-CRP was higher than ASDAS-ESR at both baseline (3.6 and 3.4, respectively) and aggregated follow-up at 6, 12, or 24 months (1.9 and 1.8, respectively). The estimated ASDAS-ESR values corresponding to the established ASDAS-CRP cut-offs were 1.4, 1.9, and 3.3. By applying these cut-offs, the proportion of discordance between disease activity states according to ASDAS-ESR and ASDAS-CRP decreased from 22.93% to 19.81% in baseline data but increased from 27.17% to 28.94% in follow-up data. CONCLUSION: We estimated the optimal ASDAS-ESR values corresponding to the established ASDAS-CRP cut-off values. However, applying the estimated cut-offs did not increase the level of agreement between ASDAS-ESR and ASDAS-CRP disease activity states to a relevant degree. Our findings did not provide evidence to reject the established cut-off values for ASDAS-ESR.

1 van der Horst Bruinsma MD PhD Rheumatology Radboud University Medical Center Nijmegen the Netherlands

A M Rodrigues MD PhD EpiDoC Unit CEDOC Nova Medical School and Rheumatology Unit Hospital dos Lusíadas Lisbon Portugal

B Michelsen MD PhD Center for Treatment of Rheumatic and Musculoskeletal Diseases Center for Rheumatology and Spine Diseases Centre for Head and Orthopaedics Copenhagen University Hospital Rigshospitalet Glostrup Denmark

D Di Giuseppe PhD Clinical Epidemiology Division Department of Medicine Solna Karolinska Institutet Stockholm Sweden

D Nordström MD PhD Departments of Medicine and Rheumatology Helsinki University Hospital Helsinki Finland

G J Macfarlane MD PhD G T Jones PhD Aberdeen Centre for Arthritis and Musculoskeletal Health University of Aberdeen Aberdeen UK

J K Wallman MD PhD Department of Clinical Sciences Lund Rheumatology Skåne University Hospital Lund University Lund Sweden

J Závada MD PhD Institute of Rheumatology Prague Czech Republic and Department of Rheumatology 1st Faculty of Medicine Charles University Prague Czech Republic

M J Santos MD PhD Department of Rheumatology Hospital Garcia de Orta Almada and Instituto Medicina Molecular Faculdade de Medicina da Universidade de Lisboa Centro Académico de Medicina de Lisboa Lisbon Portugal

M Østergaard MD PhD DMSc M L Hetland MD PhD DMSc Copenhagen Center for Arthritis Research Center for Rheumatology and Spine Diseases Centre for Head and Orthopaedics Rigshospitalet Glostrup and Department of Clinical Medicine University of Copenhagen Copenhagen Denmark

P Hellamand MD Department of Rheumatology and Clinical Immunology Amsterdam University Medical Center Amsterdam Netherlands and Amsterdam Rheumatology Immunology Center Reade and Amsterdam UMC Amsterdam the Netherlands

S A Provan MD PhD Center for Treatment of Rheumatic and Musculoskeletal Diseases Diakonhjemmet Hospital Oslo and Public Health Section Inland Norway University of Applied Sciences Elverum Norway

S Georgiadis PhD L M Ørnbjerg MD PhD Copenhagen Center for Arthritis Research Center for Rheumatology and Spine Diseases Centre for Head and Orthopaedics Rigshospitalet Glostrup Denmark

S Georgiadis PhD L M Ørnbjerg MD PhD Copenhagen Center for Arthritis Research Center for Rheumatology and Spine Diseases Centre for Head and Orthopaedics Rigshospitalet Glostrup Denmark;

T K Kvien MD PhD E K Kristianslund MD PhD Center for Treatment of Rheumatic and Musculoskeletal Diseases Diakonhjemmet Hospital Oslo Norway

Ž Rotar MD PhD K Perdan Pirkmajer MD Department of Rheumatology University Medical Centre Ljubljana and Faculty of Medicine University of Ljubljana Ljubljana Slovenia

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