Being obese or overweight may influence the blood tests used to assess the levels of inflammation for rheumatoid arthritis in women, researchers have found.

Rheumatoid arthritis is a chronic inflammatory disorder that affects joint linings, causing painful swelling. The study revealed that obesity may influence blood tests C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) -- used to diagnose and monitor rheumatoid arthritis.  Among women with rheumatoid arthritis, a greater body mass index (BMI) was associated with greater CRP, especially among women with severe obesity. 

There was also a modest association between obesity and ESR in women.  "Our results suggest that obesity may lead to increased levels of CRP and ESR in women with rheumatoid arthritis," said Michael George, from the University of Pennsylvania in the US. "The increase in these levels of inflammation was not because rheumatoid arthritis was worse in these women. In fact, we found that obesity leads to very similar increases in these lab tests even in women without rheumatoid arthritis," George added.

These links were also seen in men in the general population, but the connection between obesity and inflammation was different in men with rheumatoid arthritis. 

In these individuals, lower BMI was associated with greater CRP and ESR. 

The finding, which appear in Arthritis Care & Research, may be important for understanding the relationship between weight and inflammation and how it may differ between men and women, the researchers said. Physicians need to be careful when interpreting these lab tests since both rheumatoid arthritis and obesity can contribute to levels of inflammation. 

"Physicians might assume that high levels of inflammation mean that a patient has rheumatoid arthritis or that their rheumatoid arthritis requires more treatment when in fact a mild increase in levels of inflammation could be due to obesity instead," George explained.