RT Journal Article
SR Electronic
T1 Validating the 28-Tender Joint Count Using Item Response Theory
JF The Journal of Rheumatology
JO J Rheumatol
FD The Journal of Rheumatology
SP jrheum.110436
DO 10.3899/jrheum.110436
A1 Liseth Siemons
A1 Peter M. ten Klooster
A1 Erik Taal
A1 Ina H. Kuper
A1 Piet L.C.M. van Riel
A1 Mart A.F.J. van de Laar
A1 Cees A.W. Glas
YR 2011
UL http://www.jrheum.org/content/early/2011/09/28/jrheum.110436.abstract
AB 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.