Background: An estimated one third of asthma patients are misdiagnosed. Evaluating the difference in healthcare resource use and costs between misdiagnosed and confirmed cases of asthma can inform assessments of the burden of asthma overdiagnosis.

Methods: Participants with a self-reported physician diagnosis of asthma were recruited through random-digit dialing and followed for 1-year. The diagnosis of asthma was reassessed using bronchodilator reversibility and methacholine challenge tests. We assessed self-reported asthma-related direct healthcare costs (2017 Canadian dollars), outpatient physician visits, and medication use. We used separate regression models to measure the association between each outcome and verified asthma status, adjusting for potential confounders.

Results: The final sample included 345 individuals (61.4% female, mean age 48.9). Asthma was ruled out in 86 (24.9%) participants. Average annual direct healthcare costs for participants with confirmed asthma were $497.9 (SD $677.9), and $307.7 (SD $424.1) for participants who had asthma ruled out. In the adjusted analyses, a confirmed diagnosis was associated with higher direct healthcare costs (Relative Ratio [RR]=1.60, 95%CI 1.14-2.22), rate of specialist visits (RR=2.42, 95%CI 1.06-5.42), and reliever medication use (RR=1.62, 95%CI 1.09-2.34), but not with primary care physician visits (p=0.08), and controller medication use (p=0.12).

Conclusions: A quarter of individuals did not have asthma after objective re-evaluation. Although direct costs were higher in participants with a confirmed diagnosis of asthma, the costs of misdiagnosed asthma remained substantial. Given the high prevalence of asthma and high rate of overdiagnosis, the economic burden of overdiagnosed asthma at the population level is substantial.