Joint involvement in patients affected by systemic lupus erythematosus: application of the swollen to tender joint count ratio

Joint involvement is a common manifestation in systemic lupus erythematosus (SLE). According to the SLE disease activity index 2000 (SLEDAI-2K), joint involvement is present in case of ≥2 joints with pain and signs of inflammation. However this definition could fail to catch all the various features of joint involvement. Alternatively the Swollen to Tender joint Ratio (STR) could be used. This new index, which was originally proposed for rheumatoid arthritis (RA) patients, is based on the count of 28 swollen and tender joints. Our study is, therefore, aimed to assess joint involvement in a SLE cohort using the STR. SLE patients with joint symptoms (≥1 tender joint) were enrolled over a period of one month. Disease activity was assessed by SLEDAI-2K. We performed the swollen and tender joint count (0-28) and calculated the STR. Depending on the STR, SLE patients were grouped into three categories of disease activity: low (STR1.0). We also calculated the disease activity score based on a 28-joint count and the erythrocyte sedimentation rate (DAS28-ESR). We enrolled 100 SLE patients [F/M 95/5, mean±standard deviation (SD) age 46.3±10.6 years, mean±SD disease duration 147.1±103.8 months]. The median of tender and swollen joints was 4 (IQR 7) and 1 (IQR 2.5), respectively. The median STR value was 0.03 (IQR 0.6). According to the STR, disease activity was low in 70 patients, moderate in 23 and high in 7. A significant correlation was identified between STR values and DAS28 (r=0.33, p=0.001). The present study suggests a correlation between STR and DAS28, allowing an easier and faster assessment of joint involvement with the former index.