Reduced disability at five years with early treatment of inflammatory polyarthritis: results from a large observational cohort, using propensity models to adjust for disease severity.

OBJECTIVE To determine the effect of treatment with disease-modifying antirheumatic drugs (DMARDs) and/or steroids on 5-year disability outcome in patients with inflammatory polyarthritis. METHODS Three hundred eighty-four patients registered by the Norfolk Arthritis Register (a primary care-based observational cohort) were followed up for 5 years. Treatment details and Health Assessment Questionnaire (HAQ) scores were recorded annually. Logistic regression was used to model differences in baseline factors associated with the start of DMARDs and/or steroids within 12 months of baseline. Based on this model, each subject was given a probability of starting treatment ("propensity score"). A second model compared the odds of disability (HAQ score > or =1.00) in treated and untreated patients, adjusting for differences in disease severity using the propensity score. RESULTS Unadjusted analysis suggested that patients who received treatment had an increased odds of a worse outcome compared with those who did not receive treatment. When adjusted for differences in disease severity, using the propensity score, early treatment (within 6 months of symptom onset) was associated with a similar odds of disability at 5 years compared with those not treated (odds ratio 0.71; 95% confidence interval 0.34, 1.44). In contrast, starting treatment later (> or =6 months) was associated with a 2-fold increased odds of having a HAQ score > or =1.00 at 5 years. CONCLUSION The propensity score was a useful method of adjusting for "confounding by indication" in observational studies. Furthermore, this study showed that early treatment with DMARDs/steroids (within 6 months of symptom onset) reduced the odds of disability 5 years later to a level comparable with that of patients judged clinically as not requiring treatment.