A study of people with rheumatoid arthritis (RA) finds that those in wealthy nations are more troubled by it, even though people in poor countries have more severe symptoms.

The results, tallied from 17 countries, suggest that cultural factors may influence patients' perception of their illness, and possibly even the results of clinical trials in different locations, researchers say.

"Such differences are worrisome as they clearly point to avoidable (not caused by biologic or genetic factors) health inequities," lead study author Polina Putrik of Maastricht University Medical Center in the Netherlands said by email.

Rheumatoid arthritis, an autoimmune disease, affects the joints, connective tissues, muscle, tendons and fibrous tissue, causing pain and deformity. According to the World Health Organization, RA strikes 0.3 to 1 percent of the population.

"Patients experience a lot of anxiety about the future, the unpredictable course of their disease, side effects of medication and loss of social roles," Putrik said.

Previous studies have found international differences in patients' perception of their disease, but it's unclear how those perceptions align with disease severity, she and her colleagues say.

As reported in Annals of the Rheumatic Diseases, the researchers studied 3,920 RA patients in countries with widely varying gross domestic products, ranging from the U.S. (the richest) to Morocco (the poorest).

The average patient was 56; most were women. Doctors counted their tender and swollen joints, tested blood to measure inflammation and made an overall assessment of the patient's disease on a scale ranging from "inactive" to "severe."

Patients answered questions about their daily functioning over the past week, such as dressing and grooming, eating, walking, hygiene, errands and chores. They also rated their own overall disease state and level of fatigue.

Patients in Morocco had an average of 6.7 swollen joints, the highest count, those in the Netherlands averaged 0.9, representing the lowest count. Patients in poorer countries, on average, had higher levels of inflammation.

But patients' rating of their average daily functioning was worst in Taiwan, one of the wealthier nations. On a scale of 0 to 3, with lower scores indicating greater impairment, average scores were 0.7 in Taiwan and 1.5 in the Netherlands.

Venezuela, the third-poorest country on the list, had the lowest fatigue scores at 1.7 on a scale of zero to 10, compared to the Netherlands, the second-wealthiest country, with an average fatigue score of 5.

The differences between wealthier and poorer countries remained significant even after patient age, gender, education, other chronic diseases and medications were taken into account.

Putrik's team says patients might view their disease as worse in higher income countries because of a combination of expectations and culture.

"On the one hand, in richer countries, the best rheumatoid arthritis care is more accessible and patients may have over-optimistic expectations of the treatment," Putrik said. "On the other hand . . . patients may also experience higher pressure in terms of performing many social roles (family, work, social life) and therefore feel less satisfied with their health state."

"The finding that certain people interpret their disease differently from others is not in itself surprising," said Dr. Joshua Baker, a rheumatologist at Penn Medicine in Philadelphia. "What is really fascinating is that the wealth of the place you live can impact that interpretation."

Baker, who wasn't involved in the study, said by email that more research is needed to see whether patients' varying perspectives affect the quality and cost of RA care across the world.

"These results could be used in communication with patients to emphasize the importance of lifestyles and potential of adapting the life to the physical abilities," Putrik noted.