title: Validation of the rheumatoid arthritis diagnosis in the Swedish National Patient Register: a cohort study from Stockholm County.
abstract: BACKGROUND
  The Swedish National Patient Register offers unique possibilities for identification of large cohorts, such as patients with rheumatoid arthritis (RA). Although the overall diagnostic validity in the register has been reported as good, the aims of this study were to a) specifically validate the RA diagnosis from contemporary outpatient specialist care in this register, and b) assess the proportion of patients identified via algorithms to define incident RA in the register who in clinical practice also have new-onset disease.
  
  
  METHODS
  211 individuals with prevalent or incident RA in the National Patient Register were included. By extracting diagnosis-related parameters from their medical records, we determined if the patient fulfilled the 2010 ACR/EULAR- and the 1987 ACR-classification criteria for RA. We also determined whether clinical diagnosis was synchronous with disease onset as defined through register-based algorithms.
  
  
  RESULTS
  For 91% of the prevalent patients, the RA diagnosis in the National Patient Register fulfilled classification criteria or clinical diagnosis for RA. Among individuals identified with incident RA using a strict algorithm for new-onset disease, the RA diagnosis was substantiated in 91%, of whom 92% also represented new-onset disease.
  
  
  CONCLUSIONS
  The validity of the RA diagnosis in the National Patient Register was high and, by using specific algorithms, new-onset RA can be defined. These findings strengthen the notion that the National Patient Register may be used to define RA populations with high validity to allow for high-quality epidemiological studies.
authors.affiliation: Department of Rheumatology, Karolinska University Hospital, Stockholm, Sweden. kristin.waldenlind@karolinska.se.
authors.forename: Kristin
authors.initials: K
authors.lastname: Waldenlind
authors.forename: Jonas K
authors.initials: JK
authors.lastname: Eriksson
authors.forename: Bernhard
authors.initials: B
authors.lastname: Grewin
authors.forename: Johan
authors.initials: J
authors.lastname: Askling
mesh_terms: D000328:Adult; D000368:Aged; D001172:Arthritis, Rheumatoid / Q000175:diagnosis* / Q000453:epidemiology*; D015331:Cohort Studies; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D012042:Registries / Q000592:standards*; D012720:Severity of Illness Index; D013548:Sweden / Q000453:epidemiology