Background The disease activity score for 28 joints (DAS-28) is a widely used index measure for evaluating the disease activity of individual rheumatoid arthritis (RA) patients and for determining appropriate treatment strategies aimed at reaching or sustaining a state of remission. However, despite the frequent involvement of the 28 joints included in the DAS-28, it has been argued that the omission of the foot joints causes the DAS-28 to underestimate actual disease activity in early RA patients predominantly suffering from disease activity in the feet . Consequently, the exclusion of the foot joints remains a topic of debate and research.
Objectives To evaluate the contribution of assessing foot joints to the measurement range and measurement precision of joint counts in early RA patients using item response theory.
Methods Baseline measures of tender and swollen joint counts were analysed in 459 early RA patients who participated in the DREAM remission induction cohort. The contribution of foot joints was studied by evaluating their effect on the measurement range and measurement precision of the 28-joint counts. In addition, the alignment between the patient and joint distributions was investigated to determine whether the foot joints were informative for measuring an early RA patient’s joint tenderness or swelling.
Results Foot joints were commonly affected, with 233 patients (50.76%) experiencing tenderness and 200 patients (43.57%) experiencing swelling in at least one of the foot joints. Although foot joints appeared to be primarily informative for the measurement of joint tenderness and not so much for the measurement of joint swelling, the assessment of foot joints did not significantly improve the measurement range and measurement precision of the 28-joint counts. Furthermore, the inclusion of foot joints did not remove the existing discrepancy between the joint and patient distributions of both the 28-tender as well as the 28-swollen joint count.
Conclusions Although foot joints were frequently affected on an individual level, the inclusion of foot joints did not significantly improve the measurement range or measurement precision of the tender and swollen joint counts in early RA patients. From a measurement perspective, reduced joint counts are appropriate to use on a population level. The contribution of assessing foot joints on an individual level requires further investigation.
Bakker MF, Jacobs JWG, Kruize AA, van der Veen MJ, van Booma-Frankfort C, Vreugdenhil SA, et al. Misclassification of disease activity when assessing individual patients with early rheumatoid arthritis using disease activity indices that do not include joints of feet. Ann Rheum Dis. 2011; In Press.
Disclosure of Interest None Declared