Current treatment strategies aim to achieve clinical remission in order to prevent the long-term consequences of rheumatoid arthritis (RA). Several composite indices are available to assess remission. All of them include joint counts as the assessment of the major 'organ' involved in RA, but some employ reduced joint counts, such as the 28-joint count, which excludes ankles and feet.
The aim of the present study was to determine the relevance of excluding joints of the ankles and feet in the assessment of RA disease activity and remission. Using a longitudinal observational RA dataset, we analyzed 767 patients (80% female, 60% rheumatoid factor-positive), for whom joint counts had been recorded at 2,754 visits. We determined the number of affected joints by the 28-JC and the 32-JC, the latter including ankles and combined metatarso-phalangeal joints (as a block on each side).
Several findings were supportive of the validity of the 28-joint count: (a) Absence of joint swelling on the 28-joint scale had a specificity of 98.1% and a positive predictive value (PPV) of 94.1% for the absence of swelling also on the 32-joint scale. For absence of tender joints, the specificity and PPV were 96.1% and 91.7%, respectively. (b) Patients with swollen or tender joints in the 32-JC, despite no joint activity in the 28-JC, were clearly different with regard to other disease activity measures. In particular, the patient global assessment of disease activity was higher in these individuals. Thus, the difference in the joint count was not relevant for composite disease activity assessment. (c) The disease activity score based on 28 joints (DAS28) may reach levels higher than 2.6 in patients with feet swelling since these patients often have other findings that raise DAS28. (d) The frequency of remission did not change when the 28-JC was replaced by 32-JC in the composite indices. (e) The changes in joint activity over time were almost identical in longitudinal analysis.
The assessment of the ankles and feet is an important part in the clinical evaluation of patients with RA. However, reduced joint counts are appropriate and valid tools for formal disease activity assessment, such as done in composite indices.