Incidence of seropositive rheumatoid arthritis in population-based studies in Northern Savo, Finland, during 1980-2020.

The objective is to evaluate the incidence of seropositive rheumatoid arthritis (RA) over 40-year period in Northern Savo, Finland. Data on new seropositive RA patients according to the American College of Rheumatology (ACR) 1987 classification criteria were collected in 2020-2021. In 2020 data on tobacco exposure, patient-reported dental health and living in residences with fluoridated tap water were gathered. Incidence rates were estimated and age- and gender-adjusted to Northern Savo population. The results were compared with data acquired in studies from 1980, 1990, 2000, and 2010. In 2020, 46 seropositive RA patients (21 females and 25 males) were recorded. The crude incidence of seropositive RA fulfilling the ACR 87 criteria in 2020 was 22.3 (95% CI 16.3 to 29.8)/100 000 and age and gender-adjusted 22.3 (95% CI 15.9 to 28.8)/100 000. Tobacco exposure > 5 pack years occurred 18% of females and 56% of males. Only 16% of males were full dentate. A total of 242 incident seropositive RA (age ≥ 16 years, 55% females) were identified in all study years. No differences in the gender-specific incidence rates in each cohort or in incidence between the studies every 10 years were recorded. The incidence of seropositive RA decreased in the age group < 55 years, p = 0.003. There was no change in the incidence of seropositive RA between genders or the study years. A declining trend for occurrence of seropositive RA in the young and early middle-aged population may reflect change in risk factors.