C-reactive protein (CRP) levels are misleading in the assessment of inflammatory status in patients with rheumatoid arthritis (RA), as concentrations of the cytokine are independently influenced by levels of body fat.
"In RA, the level of CRP is frequently used in conjunction with assessments of articular swelling and tenderness to estimate the level of disease activity," note Jon Giles (Johns Hopkins University, Baltimore, Maryland, USA) and co-workers.
"In RA, inflamed synovium and circulating monocytes are sources of the cytokines that induce CRP production; however, other tissues, including adipose tissues, are known sources of these cytokines," they continue in the journal Arthritis and Rheumatism.
Giles et al attempted to establish the relative contribution of synovial and adipose tissues to circulating CRP levels by using whole-body dual X-ray absorptiometry to measure body composition in 196 patients with RA.
The authors then related these findings to data on circulating cytokine concentrations and found that level of fat mass, particularly truncal fat mass, significantly and independently predicted CRP and interleukin (IL)-6 concentrations in men, but not women.
The link between fat mass, CRP, and IL-6 level remained significant after adjusting for articular disease activity in women with RA. In contrast, increased truncal fat mass was associated with decreasing CRP and IL-6 levels only in men with RA who had significant articular involvement.
The authors note that CRP has become an established marker of inflammatory status in RA because, in contrast to the present study, previous research has shown CRP to be a significant and independent predictor of inflammation.
"This discrepancy between our findings and those from prior studies could be a by-product of our cross-sectional design," they propose.
Nevertheless, the current findings "suggest that body fat may confound the commonly ascribed associations between RA disease related factors and serum CRP concentration in certain patients, and body fat should therefore be considered when using the level of CRP for diagnostic and therapeutic decision making in RA patients," conclude Giles et al.