To estimate the incidence of early arthritis (EA) and of RA in adults (>16 yrs) in Spain.
Primary care physicians were instructed in the detection of new cases using a checklist. All cases were evaluated at EA units (EAUs) within 15 days of detection. ACR criteria for the classification of RA were assessed every 6 months thereafter.
In an area covering 4,342,378 inhabitants over the age of 16 yrs, 2467 patients were referred to the EAU, of whom 1063 fulfilled EA criteria (43.1%). After 6 months, 362 patients fulfilled RA criteria. The estimated annual incidence of EA was 25/100,000 population (95% CI: 23, 26). The annual incidence of RA was 8.3 cases/100,000 (95% CI: 7.5, 9.2): 11.3/100,000 in women (95% CI: 10.0, 12.8), and 5.2/100,000 in men (95% CI: 4.3, 6.3). The incidence of RA increased with age in both sexes. At the 6 months' assessment, 187 (51.7%) of the patients with RA were RF positive. The presentation of RA was mainly polyarticular (n = 268; 74.0%). There were 701/1063 patients with EA who did not fulfil RA criteria by 6 months after the first rheumatologist visit. If all cases of undifferentiated arthritis (n = 118; 17%) became RA, the incidence would be in the range of 10 cases/100,000 population.
RA incidence in Spain is in the lower range of published data. The incidence of EA is about three times that of RA.