Is the incidence of rheumatoid arthritis rising?: Results from Olmsted County, Minnesota, 1955–2007

Abstract

Objective: To examine trends in the incidence and prevalence of rheumatoid arthritis (RA) from 1995 to 2007. Methods: To augment our preexisting inception cohort of patients with RA (1955–1994), we assembled a population‐based incidence cohort of individuals ≥18 years of age who first fulfilled the American College of Rheumatology 1987 criteria for the classification of RA between January 1, 1995 and December 31, 2007 and a cohort of patients with prevalent RA on January 1, 2005. Incidence and prevalence rates were estimated and were age‐and sex‐adjusted to the white population in the US in 2000. Trends in incidence rates were examined using Poisson regression methods. Results: The 1995–2007 incidence cohort comprised 466 patients (mean age 55.6 years), 69% of whom were female and 66% of whom were rheumatoid factor positive. The overall age‐ and sex‐adjusted annual RA incidence was 40.9/100,000 population. The age‐adjusted incidence in women was 53.1/100,000 population (versus 27.7/100,000 population in men). During the period of time from 1995 to 2007, the incidence of RA increased moderately in women (P = 0.02) but not in men (P = 0.74). The increase was similar among all age groups. The overall age‐ and sex‐adjusted prevalence on January 1, 2005 was 0.72% (95% confidence interval [95% CI] 0.66, 0.77), which is an increase when compared with a prevalence of 0.62% (95% CI 0.55, 0.69) in 1995 (P < 0.001). Applying the prevalence on January 1, 2005 to the US population in 2005 showed that an estimated 1.5 million US adults were affected by RA. This is an increase from the previously reported 1.3 million adults with RA in the US. Conclusion: The incidence of RA in women appears to have increased during the period of time from 1995 to 2007. The reasons for this recent increase are unknown, but environmental factors may play a role. A corresponding increase in the prevalence of RA was also observed.