Being overweight can aggravate rheumatoid arthritis (RA) symptoms and make it harder to achieve remission.

But did you know that it can also make RA harder to diagnose in some cases?

A new study shows that being obese can skew RA tests due to inflammation caused by excess weight, particularly in women.

The inflammation caused by obesity can occasionally make RA disease activity appear worse than it is, leading to a misdiagnosis or an improper categorization of disease severity.

This study, out of the University of Pennsylvania, suggests that obesity can alter the results of common RA lab results like SED-rate and CRP. These blood tests are frequently used in the diagnosis and monitoring of RA.

Read more: Rheumatoid arthritis linked to mood disorders »

What researchers found

The study, published by Dr. Michael George, MSCE, of the University of Pennsylvania Health System, and his colleagues, found that there was a link between body mass index (BMI) and SED rate and CRP elevation.

In some cases, the inflammation from weight gain was what was throwing off the tests, not elevated disease activity or symptoms.

In order to deduce these results, the team of researchers looked at 2,103 people with arthritis. The researchers said the more severe the case of obesity, the bigger correlation there was between BMI and elevated RA markers.

This correlation did not always carry over to male patients.

In a statement to the press, 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.”

In the press statement, he continued, “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.”

Read more: Rigorous exercise could reduce rheumatoid arthritis symptoms »

People with arthritis not surprised

These findings were published in the medical journal Arthritis Care and Research, and some people with arthritis were not surprised.

Diana Bryan of Maryland thought she had RA. It turned out that she might not.

“I went to my primary care physician — you know, my general doctor. He ran labs and told me I had an elevated rheumatoid factor and SED rate. He said those indicate RA. I was so scared,” she told Healthline. “I went to a rheumatologist who did more tests and a physical exam. He asked my whole medical history and does not think I actually have rheumatoid. He said my inflammation could be from other factors. I’m overweight so now I’m wondering if that’s why. I do have osteoarthritis but months later after eating an anti-inflammatory diet and taking NSAIDs, plus losing some weight, my blood tests were normal, so who knows.”

“To me, it makes sense,” said Janet McKay of Pennsylvania.

McKay has RA and is also a certified nutrition coach.

“A poor diet and a high BMI can lead to inflammation. That inflammation isn’t always from an inflammatory condition like rheumatoid arthritis, though sometimes it is,” she told Healthline. “It is hard to say just how much the two are connected, but I do believe that lowering BMI is healthy for patients with RA and other chronic pain problems.”