Background: Comorbid conditions are prevalent in asthma patients but its impact on the economic burden of asthma is not well understood. To estimate the excess direct medical costs in patients with asthma, accounting for both the costs attributable to asthma and to comorbidities.

Methods: We created a propensity-score matched cohort of individuals aged 5 to 55 years between 1997 and 2012 with incident asthma and a comparison group of individuals without asthma from the health administrative data of British Columbia (BC), Canada. Sixteen major disease categories were identified using the International Classification of Diseases (ICD) codes. Excess costs [in 2013 Canadian dollars, ($)] were defined as the adjusted difference in total costs between the two groups.

Results: There were 145,742 individuals in both asthma and comparison groups. Average excess costs were $1,186.5/person-year (95% CI: 1,130.4–1,242.6) overall, of which $145.2 (143.0–147.4) were attributable to asthma and $787.7 (95% CI: 743.7, 831.7) to major comorbidity classes. Psychological disorders were the largest component of excess comorbidity costs, followed by other respiratory diseases, digestive disorders and diseases of nervous system. Comorbidity-attributable excess costs greatly increased with age but did not increase over the 10-year course of asthma.

Conclusions: In the asthma group, the excess costs attributable to comorbidity are five-times higher than costs attributable to asthma, which aggregated over age. In evaluating options for asthma management, consideration of asthma-related costs alone may result in sub-optimal policies and clinical decisions.