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Obesity may skew results of rheumatoid arthritis blood tests in women

Doctors should factor this in when using these tests, say researchers

Caroline White

Monday, 10 April 2017

Obesity may skew the results of blood tests to pick up rheumatoid arthritis — at least in women as it may prompt a rise in indicators of inflammatory activity — suggests research* published online today in Arthritis Care & Research.

The findings prompt the researchers to urge doctors to factor in obesity when using these tests.

Blood tests for C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) can help doctors assess levels of inflammation in the body. The tests may be used for diagnosing rheumatoid arthritis and for finding out how well treatment is working in those with the condition.

But previous research has linked greater body mass index (BMI) with higher CRP and ESR levels, so the researchers wanted to find out to what extent obesity might skew the results of tests for these inflammatory indicators.

They analysed CRP and ESR test results from 2103 people with rheumatoid arthritis and compared these with data from the general population.

Among women with rheumatoid arthritis and in the general population, higher BMI was associated with higher CRP levels, especially among severely obese women.

There was also a modest association between obesity and ESR in women.

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

In men with rheumatoid arthritis, lower BMI was associated with higher CRP and ESR levels. This finding may be important for understanding the association between weight and inflammation and how it may differ between the sexes, say the researchers.

“Our results suggest that obesity may lead to increased levels of CRP and ESR in women with rheumatoid arthritis,” said lead author Dr Michael George of the University of Pennsylvania Health System.

“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.”

Dr George noted that physicians need to be mindful of this association 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,” he explained.

* George MD, Giles JT, Katz PP, et al. The impact of obesity and adiposity on inflammatory markers in patients with rheumatoid arthritis. Arthritis Care & Research, accepted manuscript online: 10 April 2017. DOI: 10.1002/acr.23229

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