Estimating the incidence of rheumatoid arthritis - Trying to hit a moving target?

Objective. To examine the effect of delay between symptom onset and notific ation to an arthritis register and the effect of application of the America n College of Rheumatology (ACR; formerly, the American Rheumatism Associati on) 1987 criteria in a cumulative manner on estimates of the incidence of r heumatoid arthritis (RA). Methods, General practitioners and/or hospital consultants in the Norwich H ealth Authority, Norfolk, UK, notified the Norfolk Arthritis Register (NOAR ) of all patients who had onset of inflammatory polyarthritis (swelling of greater than or equal to 2 joints) during 1990, The patients were assessed within 2 weeks of notification and annually thereafter. The ACR 1987 criter ia for RA were applied at each assessment. Age- and sex-specific incidence rates were calculated. Results. If up to 12 months elapsed from symptom onset to notification to N OAR and the ACR criteria were applied at the baseline assessment, RA incide nce estimates, age-adjusted to the population of England and Wales, were 30 .8/100,000 for women and 12.7/100,000 for men. If up to 5 years elapsed fro m symptom onset to notification, these estimates rose by 45% for women and 36% for men. If up to 5 years elapsed between symptom onset and notificatio n and the criteria were applied cumulatively, the estimates rose by 75% and 93% for women and men, respectively, compared with the 1-year data, reachi ng 54.0/100,000 for women and 24.5 per 100,000 for men. Conclusion, Accurate estimation of the incidence of RA requires long-term f ollowup of patients who present with undifferentiated inflammatory polyarth ritis, The highest age-adjusted estimates from this study are probably the best that are available.