Objective: To estimate the a) prevalence of polymyositis and dermatomyositis using population-based administrative data; b) sensitivity of case ascertainment approaches; c) patient demographics and these parameters.
Methods: We ascertained cases from Quebec physician billing and hospitalization databases (approximately 7.5 million beneficiaries). We compared three different case definition algorithms and also used statistical methods that account for imperfect case ascertainment to generate estimates of disease prevalence and of case ascertainment sensitivity. We developed a hierarchical Bayesian latent class regression model, assessing patient characteristics with respect to these parameter estimates.
Results: Using methods that account for the imperfect nature of both billing and hospitalization databases, we estimated the 2003 prevalence of polymyositis and dermatomyositis to be 21.5/100,000 (95% credible interval [CrI] 19.4, 23.9). Prevalence was higher for women and for older individuals, with a tendency for higher prevalence in urban areas. Prevalence estimates were lowest in young rural men (2.7/100,000, 95 CrI 1.6, 4.1) and highest in older urban women (70/100,000, 95% CrI 61.3, 79.3). Sensitivity of case ascertainment tended to be lower for older versus younger individuals, particularly for rheumatology billing data. Billing data appeared more sensitive in ascertaining cases in urban (versus rural) regions, while hospitalization data seemed most useful in rural areas.
Conclusions: We found marked variations in the prevalence of polymyositis and dermatomyositis according to age, sex, and region. Our methods allow adjustment for the imperfect nature of multiple data sources, and estimation of the sensitivity of different case ascertainment approaches.