Are the Incidence and Prevalence of RA Changing in the United States?

Map

The epidemiology of rheumatoid arthritis (RA) is influenced by a number of factors, including survival, changes in diagnostic tools, and temporal trends in environmental factors that may promote or protect from disease.  Here, Myasoedova et al (Arthritis Rheum 2010; 62(6): 1576) explore the trends in RA incidence and prevalence over 50 years in a population-based cohort study conducted in Olmsted County, Minnesota.

Methods

Linked records for participants included in the Rochester Epidemiology Project, which includes nearly all of the inhabitants of Olmsted County, Minnesota, were examined for RA diagnosed between 1995 and 2007.  Questionable cases were adjudicated by the investigators.   Incidence and prevalence estimates were tracked over time and added to prior estimates collected for the period 1955-1994.  Incidence rates were age and gender adjusted to the population of U.S. Caucasians from 2000.

Results

Among 1,761 arthritis diagnoses in adults between 1995 and 2007, 466 were confirmed as having RA.  Most were female (69%) with a mean age of 56 years at diagnosis.  Rheumatoid factor (RF) was present in 66% and 20% had evidence for erosions within the first year after diagnosis.  Compared to prior periods, smoking in men, but not women, had decreased from the 1985-1994 period.  The demographic adjusted incidence rate for RA was 41 per 100,000 (95% CI 37, 45), 53 per 100,000 in women and 28 per 100,000 in men.  The maximum incidence was in the age group 65-74 years of age.  Over the decade studied, RA incidence increased 2.5% per year in women (95% CI 0.3, 4.7%); however, there was no significant change in men.  There was no difference in incidence rate trends according to RF status or age at diagnosis.  Accordingly, the prevalence of RA increased in women (0.98% compared to the estimate of 0.77% from the previous decade), but was unchanged in men.   From these prevalence rates, there are an estimated 1.5 million U.S. adults with RA.

Conclusions

Recent trends in RA incidence from Rochester County, Minnesota indicate an increase in women, but no change in men.

Editorial Comment

These are interesting findings, considering that prior estimates had indicated a decline in RA incidence over the preceding several decades.  The reasons behind the increase in women are unclear.  Smoking is a strong risk factor for RA, and rates have not declined in U.S. women compared to men.  Thus, it is possible that smoking could be the environmental risk factor implicated in the rise in RA incidence described here; however, other factors may contribute.  Another explanation for the increase may be shifts in diagnosis, particularly the additional of anti-CCP antibodies to clinical practice.  Shifts in diagnosis may result in including patients with milder disease who may not have been classified previously with RA; however, one would expect changing trends in diagnosis to affect men and women equally.

Jon Giles, MD

About Jon Giles, MD