title: Validating the 28-tender joint count using item response theory.
abstract: OBJECTIVE
  To examine the construct validity of the 28-tender joint count (TJC-28) using item response theory (IRT)-based methods.
  
  
  METHODS
  A total of 457 patients with early stage rheumatoid arthritis (RA) were included. Internal construct validity of the TJC-28 was evaluated by determining whether the TJC-28 fit a 2-measure logistic IRT model. As well, we tested whether the discrimination and difficulty parameters of the joints properly reflected the known left-right symmetry of joint involvement. External validity was evaluated by correlations with other established measures of disease activity, including pain, disability, general health, erythrocyte sedimentation rate (ESR), and the 28-swollen joint count.
  
  
  RESULTS
  The TJC-28 showed a good fit with the 2-parameter logistic model, with no relevant differential item functioning across sex, age, and time and with excellent reliability. The 28 joints covered a reasonable range of disease activity, even though they were mainly targeted at patients with moderate or high disease activity levels. The joint parameters reflected the left-right symmetry of joint involvement for all pairs of joints except one. All disease activity measures, except ESR, were significantly correlated with the TJC-28. Most correlations were of the expected magnitude.
  
  
  CONCLUSIONS
  The TJC-28 showed good internal and acceptable external construct validity for patients with early-stage RA. The IRT analyses did point to some potential limitations of the instrument, a major problem being its limited measurement range. Future research should examine whether instrument modifications might lead to a more robust assessment of disease activity in patients with RA.
authors.affiliation: Arthritis Center Twente, Department of Psychology, Health and Technology, University of Twente, Enschede, The Netherlands. l.siemons@utwente.nl
authors.forename: Liseth
authors.initials: L
authors.lastname: Siemons
authors.forename: Peter M
authors.initials: PM
authors.lastname: ten Klooster
authors.forename: Erik
authors.initials: E
authors.lastname: Taal
authors.forename: Ina H
authors.initials: IH
authors.lastname: Kuper
authors.forename: Piet L C M
authors.initials: PL
authors.lastname: van Riel
authors.forename: Mart A F J
authors.initials: MA
authors.lastname: van de Laar
authors.forename: Cees A W
authors.initials: CA
authors.lastname: Glas
mesh_terms: D000328:Adult; D000368:Aged; D001172:Arthritis, Rheumatoid / Q000175:diagnosis* / Q000473:pathology* / Q000503:physiopathology; D019937:Diagnostic Techniques and Procedures / Q000592:standards*; D005260:Female; D006801:Humans; D007596:Joints / Q000473:pathology* / Q000503:physiopathology; D008297:Male; D008875:Middle Aged; D010146:Pain / Q000503:physiopathology; D010147:Pain Measurement; D012074:Remission Induction; D015203:Reproducibility of Results; D012720:Severity of Illness Index