TY - JOUR T1 - Validating the 28-Tender Joint Count Using Item Response Theory JF - The Journal of Rheumatology JO - J Rheumatol DO - 10.3899/jrheum.110436 SP - jrheum.110436 AU - Liseth Siemons AU - Peter M. ten Klooster AU - Erik Taal AU - Ina H. Kuper AU - Piet L.C.M. van Riel AU - Mart A.F.J. van de Laar AU - Cees A.W. Glas Y1 - 2011/10/01 UR - http://www.jrheum.org/content/early/2011/09/28/jrheum.110436.abstract N2 - 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. ER -