Background The development of new classification criteria for rheumatoid arthritis (RA) calls for a re-estimation of RA incidence rates. The new criteria purport to have increased sensitivity to identify RA in patients with early inflammatory arthritis (EIA).
Objectives To estimate the age and sex-specific incidence of RA in Norfolk, England using the new criteria
Methods This study included all patients aged ≥16 notified to the Norfolk Arthritis Register (NOAR), a primary-care based inception cohort of patients with EIA, from 1990-5 with symptom onset in 1990. The Norwich Health Authority population based on the 1991 census was the denominator. Age and sex specific incidence rates using both 1987 and 2010 classification criteria were calculated at baseline visit, annually for the first 3 years and at 5 years. At each follow up both criteria sets were applied cross-sectionally and cumulatively, i.e. once a patient had satisfied a particular criterion that result was carried forward to future assessments.
Results 283 patients were notified to NOAR with symptom onset in 1990. 23 were excluded as an alternative diagnosis was made by their rheumatologist. The overall incidence rate (IR) when applying the 2010 criteria at baseline was 54/100 000 for women and 25/100 000 for men, compared to 44/100 000 for women and 19/100 000 for men using 1987 criteria at baseline. Age and sex-specific IRs using the 2010 classification criteria at baseline were similar to cumulative IRs applying the 1987 criteria up to 5 years. IRs using both criteria sets converged when applied cumulatively. However, 10 patients (4%) satisfied the 1987 criteria cumulatively after 5 years but never satisfied the new criteria, and 37 (14%) satisfied the new criteria but never satisfied the 1987 criteria.
Conclusions The 2010 classification criteria aim to identify early, those patients with EIA who, in the absence of appropriate treatment, would go on to develop persistent, erosive and disabling RA. This study shows that the new criteria classify at baseline similar numbers of patients as having RA, as the previous criteria would have taken up to 5 years to identify. A small proportion of patients (18%) satisfied only one set of classification criteria over 5 years. Those patients who satisfied the 1987 criteria but not the 2010 had more hand joint involvement, symmetrical arthritis and morning stiffness. These results provide important information for health economics evaluation and healthcare planning.
Disclosure of Interest None Declared