Objective: The C-reactive protein (CRP) level and erythrocyte sedimentation rate (ESR) are important disease activity biomarkers in rheumatoid arthritis (RA). This study aimed to determine to what extent obesity biases these biomarkers. Methods: Body mass index (BMI) associations with CRP level and ESR were assessed in 2 RA cohorts: the cross-sectional Body Composition (BC) cohort (n = 451), including whole-body dual x-ray absorptiometry measures of fat mass index; and the longitudinal Veterans Affairs Rheumatoid Arthritis (VARA) registry (n = 1,652), using multivariable models stratified by sex. For comparison, associations were evaluated in the general population using the National Health and Nutrition Examination Survey. Results: Among women with RA and in the general population, greater BMI was associated with greater CRP levels, especially among women with severe obesity (P < 0.001 for BMI ≥35 kg/m2 versus 20–25 kg/m2). This association remained after adjustment for joint counts and patient global health scores (P < 0.001 in BC and P < 0.01 in VARA), but was attenuated after adjustment for fat mass index (P = 0.17). Positive associations between BMI and ESR in women were more modest. In men with RA, lower BMI was associated with higher CRP levels and ESR, contrasting with positive associations among men in the general population. Conclusion: Obesity is associated with higher CRP levels and ESR in women with RA. This association is related to fat mass and not RA disease activity. Low BMI is associated with higher CRP levels in men with RA; this unexpected finding remains incompletely explained but likely is not a direct effect of adiposity.
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