The evaluation of disease activity in obese rheumatoid arthritis (RA) patients presents challenges particularly in the clinical assessment of swollen joints. This study examines the effect of obesity on the American College of Rheumatology (ACR) core set measures used in assessing RA disease activity with specific focus on the swollen joint count (SJC).
We examined a cross-sectional cohort of 323 early seropositive RA patients (symptom duration ≤15 months). Patients were biologic-naive with equal to or more than 6/44 SJC and equal to or more than 9/44 tender joint count. The ACR core set measures, components of Disease Activity Score (DAS) 44/erythrocyte sedimentation rate (ESR), DAS28/ESR4 item, Clinical Disease Activity Index (CDAI), and body mass index (BMI) were collected. Disease activity measures were compared between BMI categories. Multivariable linear regression models assessed the relationship between high BMI (≥30 kg/m) and lower-extremity (LE) SJC and SJC44 while accounting for other ACR measures.
Disease Activity Score 44/ESR4 item, Health Assessment Questionnaire Disability Index, physician global, and SJC44 differed across BMI categories (p < 0.05). Of the SJC44, metacarpophalangeal joints and LE joints (knees, ankles, metatarsophalangeal joints) were associated with increased swelling in all BMI groups (P < 0.05). Obesity was significantly associated with LE SJC after adjusting for ACR core set measures.
There is a direct association between increased BMI and increased swelling of LE joints in RA patients. Increases in DAS44-measured disease activity are higher in obese RA patients because of increased LE swollen joints. Disease Activity Score 28 and Clinical Disease Activity Index, which emphasize upper-extremity joint assessment, are not significantly influenced by obesity.