title: Validity of ankylosing spondylitis and undifferentiated spondyloarthritis diagnoses in the Swedish National Patient Register.
abstract: OBJECTIVE
  Epidemiological studies of spondyloarthritis (SpA), using ICD codes from the Swedish National Patient Register (NPR), offer unique possibilities but hinge upon an understanding of the validity of the codes. The aim of this study was to validate the ICD codes for ankylosing spondylitis (AS) and undifferentiated SpA (uSpA) in the NPR against the established classification criteria [modified New York (mNY), Assessment of SpondyloArthritis international Society (ASAS), Amor, and European Spondyloarthropathy Study Group (ESSG) criteria].
  
  
  METHODS
  All patients with an ICD-8/9/10 code of AS or uSpA in the NPR 1966-2009 at a visit to a specialist in rheumatology or internal medicine or corresponding hospitalization, alive and living in Sweden 2009, were identified (n=20,089). Following a structured procedure to achieve geographical representativeness, 500 random patients with a diagnosis of AS or uSpA in 2007-2009 were selected. Based on a structured review of clinical records, positive predictive values (PPVs) for fulfilling the criteria sets were calculated.
  
  
  RESULTS
  For those having received an ICD code for AS, the PPVs for fulfilling the mNY criteria or any set of SpA criteria were 70% and 89%, respectively. For those with an uSpA diagnosis (and never an AS diagnosis), the corresponding PPVs were 20% and 79%. The subset with both AS and uSpA diagnoses (overlap=12%) were as likely to fulfil the mNY criteria as the group that had been coded as AS only.
  
  
  CONCLUSIONS
  The diagnosis codes for AS or uSpA had high PPVs, suggesting that our case identification in the Swedish NPR can be used for nationwide, population-based, epidemiological studies of these diseases.
authors: a Department of Rheumatology and Inflammation Research , Sahlgrenska Academy at the University of Gothenburg , Gothenburg.
authors: U
authors: U
authors: Lindström
authors: b Section of Rheumatology, Department of Clinical Sciences , Lund University , Malmö
authors: S
authors: S
authors: Exarchou
authors: c Department of Rheumatology , Falun , Hospital , Falun.
authors: V
authors: V
authors: Sigurdardottir
authors: d Department of Public Health and Clinical Medicine , Rheumatology, Umeå University , Umeå
authors: B
authors: B
authors: Sundström
authors: e Clinical Epidemiology Unit, Department of Medicine Solna , Karolinska Institutet , Stockholm , Sweden.|f Rheumatology Unit, Department of Medicine Solna , Karolinska Institutet , Stockholm , Sweden.
authors: J
authors: J
authors: Askling
authors: e Clinical Epidemiology Unit, Department of Medicine Solna , Karolinska Institutet , Stockholm , Sweden.
authors: J K
authors: JK
authors: Eriksson
authors: a Department of Rheumatology and Inflammation Research , Sahlgrenska Academy at the University of Gothenburg , Gothenburg.
authors: H
authors: H
authors: Forsblad-d'Elia
authors: b Section of Rheumatology, Department of Clinical Sciences , Lund University , Malmö
authors: C
authors: C
authors: Turesson
authors: b Section of Rheumatology, Department of Clinical Sciences , Lund University , Malmö|g The Parker Institute, Department of Rheumatology; Copenhagen University Hospital , Frederiksberg , Denmark.
authors: L E
authors: LE
authors: Kristensen
authors: a Department of Rheumatology and Inflammation Research , Sahlgrenska Academy at the University of Gothenburg , Gothenburg.
authors: L
authors: L
authors: Jacobsson
mesh_terms: D000293:Adolescent; D000328:Adult; D059019:Clinical Coding; D015331:Cohort Studies; D005260:Female; D006801:Humans; D015994:Incidence; D008297:Male; D012042:Registries; D012189:Retrospective Studies; D025241:Spondylarthritis / Q000145:classification / Q000175:diagnosis* / Q000453:epidemiology*; D013167:Spondylitis, Ankylosing / Q000145:classification / Q000175:diagnosis* / Q000453:epidemiology*; D013548:Sweden / Q000453:epidemiology; D055815:Young Adult