Introduction: There is little information on recent trends in the economic burden of asthma. Our objective was to estimate the excess costs of asthma and their trend in British Columbia, Canada, from 2002 to 2011.

Methods: A retrospective cohort of individuals aged 5 to 55 years was constructed from the provincial administrative health databases, consisting of patients with physician-diagnosed asthma and a propensity-score-matched comparison sample from the general population. Total direct medical costs were calculated as the sum of hospitalisations, outpatient visits, and medication costs, adjusted to 2012 Canadian dollars ($). Excess costs were defined as the difference in costs between the asthma and comparison groups.

Results: 341,457 individuals (mean age at entry 27.3, 54.1% female) were equally divided into the asthma and comparison groups. Excess costs in patients with asthma were $1,028.0 (95%CI $982.7-$1073.4) per patient-year. Medications contributed to the greatest share of excess costs ($471.7/patient-year), whereas hospitalisation and outpatient costs were, respectively, $272.2/patient-year and $284.1/patient-year. Only $192.9/patient-year were attributable to asthma itself. There was a 2.9%/year increase in excess costs (P<0.001), a combination of asthma-attributable costs declining by 0.8%/year while non-asthma excess costs increasing by 3.8%/year. The most dramatic trend was observed in asthma-related outpatient costs, which decreased by %6.6/year.

Conclusions: A significant share of excess costs in asthma is not attributable to the disease itself. The pattern of costs changed significantly during the study period. The burden of comorbid conditions should be considered in developing evidence-based policies for management of patients with asthma.