To estimate the incidence of rheumatoid arthritis (RA) in primary care and to investigate associations with consultation behaviour, risk factors, and comorbidities, using the UK General Practice Research Database (GPRD).
Subjects with a first-ever diagnosis of RA between 1 January 1996 and 31 December 1997 (n = 579) were identified from a cohort of 1 206 918 subjects aged 20-79 years without cancer. Controls from the same cohort were frequency-matched to the RA group by age, sex, and calendar year (n = 4234). Odds ratios (ORs) and 95% confidence intervals (CIs) of being diagnosed with RA in association with a range of factors were estimated using logistic regression analysis.
RA incidence was 0.15 per 1000 person-years, was higher in women than in men, and increased with age in both sexes. Consultations and use of non-steroidal anti-inflammatory drugs (NSAIDs) prior to diagnosis were increased in subjects with RA. An increased risk of RA was observed in association with anaemia in the previous year (OR 2.63, 95% CI 1.54-4.48) and with smoking (1.33, 1.07-1.67). A decreased risk of RA was observed in association with infectious diseases (0.68, 0.50-0.94) and pregnancy in the previous year (0.22, 0.06-0.77), diabetes (0.45, 0.26-0.78), and hypertension (0.74, 0.57-0.94). We found no association with alcohol intake, obesity, or use of low-dose aspirin, oral contraceptives, or hormone replacement therapy (HRT).
Smoking was identified as the only significant lifestyle-related risk factor for RA. Infection in the previous year was associated with a reduced likelihood of RA.