Contribution of assessing forefoot joints in early rheumatoid arthritis patients: Insights from item response theory.

OBJECTIVE: To evaluate the contribution of assessing forefoot joints to the measurement range and measurement precision of joint counts in early RA using item response theory. METHODS: Baseline measures of tender and swollen joint counts were analysed in 459 early RA patients from the DREAM remission induction cohort. The contribution of forefoot joints was studied by evaluating their effect on the measurement range and measurement precision of measures based on 28-joint counts. In addition, the alignment between the patient and joint distributions was investigated to determine whether the forefoot joints were informative for measuring an early RA patient's joint tenderness or swelling. RESULTS: In total, 233 patients (50.76%) experienced tenderness and 200 patients (43.57%) experienced swelling in ≥ 1 forefoot joint. Forefoot joints were more informative for measuring joint tenderness than joint swelling, but did not significantly improve the measurement range and measurement precision of the 28-joint counts. Furthermore, including forefoot joints did not remove the existing discrepancy between the joint and patient distributions in both joint counts. CONCLUSIONS: Forefoot joints were frequently affected on an individual level, but did not significantly improve the measurement range or precision of 28-joint counts in early RA patients. From a measurement perspective, reduced joint counts are appropriate for use on a population level. The contribution of assessing forefoot joints on an individual level requires further investigation. Additionally, the results should be cross-validated in patients with longer disease duration, to determine whether the pattern of joint involvement is similar in later stages of RA.