Obesity influences rheumatoid arthritis diagnostic tests

Obesity causes patients to receive higher scores in blood tests used to diagnose rheumatoid arthritis even if they do not have the condition, researchers have warned.

Obese patients have higher levels of C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR), regardless of whether they have rheumatoid arthritis, researchers have found.

The study published in Arthritis Care & Research, said fat mass could push up CRP and ESR levels, which could be mistaken for arthritis disease activity.

Blood tests to determine CRP and ESR levels are used to assess levels of inflammation in the body, which in turn is used to diagnose rheumatoid arthritis or determine how effective treatment is.

Over 2,100 patients with rheumatoid arthritis were involved in the trial, which was compared with data from the general population.

Among women in the general population and those who had rheumatoid arthritis, CRP levels increased in line with BMI, becoming especially high in those with severe obesity, the researchers found. ESR levels also increased, but at a more modest rate.

In men with rheumatoid arthritis, lower BMI was associated with greater CRP and ESR, contrasting positive associations among men in the general population – who followed the same pattern as women.

Clinicians should keep the association in mind when testing patients, the researchers said.

Rheumatoid arthritis

Lead author Dr Michael George said: ‘Our results suggest that obesity may lead to increased levels of CRP and ESR in women with rheumatoid arthritis.

‘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.

‘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.’

GPC clinical and prescribing lead Dr Andrew Green said doctors should look at changes over time in CRP and ESR values to diagnose patients and take into account individual differences.

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