Validating the 28-tender joint count using item response theory

Liseth Tjin-Kam-Jet-Siemons, Peter M. ten Klooster, Erik Taal, I.H. Kuper, Piet L.C.M. van Riel, Mart A F J van de Laar, Cornelis A.W. Glas

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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. Conclusion: 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.
Original languageEnglish
Pages (from-to)2557-2564
JournalJournal of rheumatology
Issue number12
Publication statusPublished - 2011


  • IR-82754
  • METIS-281285


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