We investigated the association between serum levels of C-reactive protein (CRP) and the extent of multijoint pain among individuals with hip/knee osteoarthritis (OA) and determined whether the association differs by sex.

Serum CRP and cartilage oligomeric matrix protein (COMP) were determined by enzyme-linked immunosorbent assay (ELISA) in 189 individuals (101 female, 88 male) scheduled for total hip/knee arthroplasty for OA. Patients indicated on a homunculus all painful joints; a summed count was derived. A series of negative binomial regression models was used to investigate the cross-sectional association between painful joint count (outcome) and serum CRP concentrations, adjusting for age, sex, body mass index (BMI), comorbidity count and COMP. An interaction between sex and these biomarkers was tested.

Mean age: 66 among women, 65 among men. Women had higher mean joint count (3.7 vs 2.5, P < 0.01; 4+ joint count reported by 37% women, 25% men). Median CRP concentration was higher in women (15.4 mg/l vs 9.3, P = 0.07). From adjusted analyses, the effects of both ln(CRP) and ln(COMP) were modified by sex (P < 0.05). Increasing ln(CRP) was associated with greater painful joint count among women, but not men.

There may be a dose-response association between painful joint burden in OA and systemic inflammation, and it appears the association is sex-specific, which may in part explain inconsistent findings in the literature. Our results underline the importance of showing sex-specific associations in OA, especially when studying the influence of inflammation.