Obesity may influence rheumatoid arthritis blood tests

The effect of obesity on C-reactive protein may alter its performance as a biomarker of disease activity in clinical trials and in practice, authors said.

Obesity in women may influence C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR), affecting the use of these tests to diagnose and monitor rheumatoid arthritis, a study found.

Researchers looked at whether obesity influences these markers among 2,103 individuals with rheumatoid arthritis and compared it with data from the general population. Body-mass index (BMI) associations with the two blood markers were assessed using the cross-sectional Body Composition (BC) cohort (n=451) including whole-body dual-energy 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 appeared April 10 in Arthritis Care & Research.

Among women with rheumatoid arthritis and in the general population, greater BMI was associated with greater CRP, especially among women with severe obesity (all P<0.001 for BMI ≥35 vs. 20 to 25 kg/m2). This association remained after adjustment for joint counts and patient global (P<0.001 in BC and <0.01 in VARA), but was attenuated after fat mass index adjustment (P=0.17).There was also a modest association between obesity and ESR in women.

These links were also seen in men in the general population, the authors noted, but the connection between obesity and inflammation was different in men with rheumatoid arthritis, in whom lower BMI was associated with higher CRP and ESR.

Study authors cautioned that physicians might assume that high levels of inflammation, as indicated by these tests, mean that a patient has rheumatoid arthritis or that rheumatoid arthritis requires more treatment when the mild increase in levels of inflammation could be due to obesity instead.

“The effect of obesity on CRP may alter its performance as a biomarker of disease activity in clinical trials and in practice,” the authors wrote. “While our conservative assessment suggests a modest effect of obesity on DAS28-CRP (0.15 to 0.30 units, on average), correction of CRP values did result in the reclassification of disease activity in approximately 10% of severely obese women. Furthermore, because obesity may be expected to impact the reliability of other component measures of disease activity, clinicians may tend to rely more on objective measures such as CRP for diagnosis or to make treatment decisions.”