To examine trends in the incidence of rheumatoid arthritis (RA) from 2005 to 2014 overall and by serological status as compared with 1995-2004 and 1985-1994.
We evaluated RA incidence trends in a population-based inception cohort of individuals aged 18 years who first fulfilled the 1987 American College of Rheumatology (ACR) criteria for RA between 1 January 1985 and 31 December 2014. Incidence rates were estimated and were age-adjusted and sex-adjusted to the white population in the USA in 2010. Trends in incidence were examined using Poisson regression methods.
The 2005-2014 incidence cohort comprised 427 patients: mean age 55.4 years, 68% female, 51% rheumatoid factor (RF) positive and 50% anti-cyclic citrullinated peptide antibody positive. The overall age-adjusted and sex-adjusted annual RA incidence in 2005-2014 was 41/100 000 population (age-adjusted incidence: 53/100 000 in women and 29/100 000 in men). While these estimates were similar to the 1995-2004 decade, there was a decline in the incidence of RF-positive RA in 2005-2014 compared with the previous two decades (p=0.004), with a corresponding increase in RF-negative cases (p<0.001). Smoking rates declined and obesity rates increased from earlier decades to more recent years.
Significant increase in incidence of RF-negative RA and decrease in RF-positive RA in 2005-2014 compared with previous decades was found using 1987 ACR criteria. The incidence of RA overall during this period remained similar to the previous decade. The changing prevalence of environmental factors, such as smoking, obesity and others, may have contributed to these trends. Whether these trends represent a changing serological profile of RA requires further investigation.

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