PT  - JOURNAL ARTICLE
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
TI  - Validating the 28-Tender Joint Count Using Item Response Theory
AID  - 10.3899/jrheum.110436
DP  - 2011 Oct 01
TA  - The Journal of Rheumatology
PG  - jrheum.110436
4099  - http://www.jrheum.org/content/early/2011/09/28/jrheum.110436.short
4100  - http://www.jrheum.org/content/early/2011/09/28/jrheum.110436.full
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.