Impact of the size and number of swollen joints on serum C-reactive protein level and erythrocyte sedimentation rate in rheumatoid arthritis: a cross-sectional study in Japan.

No studies have yet reported the influence of swelling in individual joints on serum C-reactive protein and erythrocyte sedimentation rate. To examine this association, we used data from the NinJa registry, the largest registry of rheumatoid arthritis patients in Japan. Sixty-six palpable joints were categorized by size into three groups (small, medium-sized, and knees) with surface area cutoffs of 10 and 100 cm2. Of 10,720 cases registered in NinJa in 2012, 8444 cases with either no swollen joints or swelling limited to one joint-size category were analyzed. Groups with larger numbers of swollen joints showed higher levels of both markers in each joint-size category. Groups with larger swollen joints had higher levels of both markers compared with groups with the same number of (smaller) swollen joints. Linear regression revealed that the increments of C-reactive protein (mg/dL/joint) and erythrocyte sedimentation rate (mm/1 h/joint) were 0.056 and 0.89, 0.24 and 5.0, and 0.46 and 8.9 for small and medium-sized joints and knee joints, respectively. The levels of systemic inflammation markers increased with the involvement of larger and/or more joints. These results were successfully illustrated by the use of large-scale data, which eliminated wide intragroup scattering of the marker values.