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 Dec 01 TA - The Journal of Rheumatology PG - 2557--2564 VI - 38 IP - 12 4099 - http://www.jrheum.org/content/38/12/2557.short 4100 - http://www.jrheum.org/content/38/12/2557.full SO - J Rheumatol2011 Dec 01; 38 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.