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Else Helene Ibfelt,1 Jan Sørensen,2,3 Dorte V Jensen,4,5 Lene Dreyer,5,6 Berit Schiøttz‑Christensen,7 Pia H Thygesen,8 Ada Colic,9 Johnny L Raun,10 Natalia Manilo,11 Anne Rødgaard,4 Uta E Poulsen,12 Claus Rasmussen,13 Torben Hansen,14 Babara Unger,15 Randi Pelck,16 Anita Kincses,17 Henrik Nordin,18 Tove Lorenzen,19 Ali Theibich,20 Inger Marie Jensen Hansen,21 Jakob Espesen,22 Jolanta Grydehøj,23 Mette Holland-Fischer,24 Anne Gitte Loft,25 Merete Lund Hetland4,26 1Research Centre for Prevention and Health, Capital Region of Denmark, Rigshospitalet – Glostrup, Denmark; 2Centre of Health Economics Research, Institute of Public health, University of Southern Denmark, Odense, Denmark; 3Healthcare Outcome Research Centre, Royal College of Surgeons in Ireland, Dublin, Ireland; 4DANBiO and Copenhagen Center for Arthritis Research, Center for Rheumatology and Spine Diseases, Rigshospitalet – Glostrup, Glostrup, 5Center for Rheumatology and Spine Diseases, Gentofte University Hospital, Hellerup, 6The Parker Institute, Frederiksberg and Bispebjerg Hospital, Frederiksberg, 7Spine Centre of Southern Denmark, Hospital Lillebaelt, Middelfart, 8Department of Rheumatology, Odense University Hospital, Odense, 9Department of Rheumatology, Sydvestjysk Sygehus, Esbjerg/Grintsted, 10Department of Internal Medicine and Rheumatology, SLB – Fredericia Hospital, Fredericia, 11Department of Rheumatology, Frederiksberg Hospital, Copenhagen, 12Department of Rheumatology, Gigthospital Gråsten, Gråsten, 13Clinic of Internal Medicine, Rheumatology, Regionshospital Nordjylland, Hjørring, 14Department of Rheumatology, Holbæk sygehus, Holbæk, 15Department of Internal Medicine/Rheumatology, Hospitalsenheden Horsens, Horsens, 16Department of Rheumatology, Zealand University Hospital, Køge, 17Department of Rheumatology, Nordsjællands Hospitaler, Hillerød, 18Center for Rheumatology and Spine Diseases, Centre of Head and Orthopedics, Rigshospitalet, Copenhagen, 19Department of Rheumatology, Silkeborg Regional Hospital and University Clinic, Silkeborg, 20Department of Rheumatology, Slagelse Hospital, Slagelse, 21Department of Rheumatology and University of Southern Denmark, Svendborg Hospital, Odense, 22Department of Internal Medicine, Vejle Hospital, Vejle, 23Department of Rheumatology, Regional Hospital, West Jutland, Herning, 24Department of Rheumatology, Aalborg University Hospital, Ålborg, 25Department of Rheumatology, Aarhus University Hospital, Aarhus, 26Department of Clinical Medicine, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark Objectives: In Denmark, patients with rheumatoid arthritis (RA) are registered in the nationwide clinical DANBIO quality register and the Danish National Patient Registry (DNPR). The aim was to study the validity of the RA diagnosis and to estimate the completeness of relevant RA cases in each registry.Study design and setting: Patients registered for the first time in 2011 with a diagnosis of RA were identified in DANBIO and DNPR in January 2013. For DNPR, filters were applied to reduce false-positive cases. The diagnosis was verified by a review of patient records. We calculated the positive predictive values (PPVs) of the RA diagnosis registrations in DANBIO and DNPR, and estimated the registry completeness of relevant RA cases for both DANBIO and DNPR. Updated data from 2011 to 2015 from DANBIO were retrieved to identify patients with delayed registration, and the registry completeness and PPV was recalculated.Results: We identified 1,678 unique patients in DANBIO or in DNPR. The PPV (2013 dataset) was 92% in DANBIO and 79% in DNPR. PPV for DANBIO on the 2015 update was 96%. The registry completeness of relevant RA cases was 43% in DANBIO, increasing to 91% in the 2015 update and 90% in DNPR.Conclusion: DANBIO held a high proportion of true RA cases (96%) and was found to be superior to the DNPR (79%) with regard to the validity of the diagnosis. Both registries were estimated to have a high completeness of RA cases treated in hospital care (~90%). Keywords: rheumatoid arthritis, validity, incidence, clinical registry, Denmark