BACKGROUND: The majority of asthma patients have mild disease. However, the clinical course of mild asthma and risk factors for progression are not well studied.

OBJECTIVE: To examine the long-term trajectories of mild asthma and the effects of early-stage risk factors on the subsequent disease course.

METHODS: From the administrative health data of British Columbia, Canada, we identified patients 14 to 45 years of age with newly diagnosed mild asthma between January 1997 and December 2012. For each follow-up year, we categorized a patient's asthma severity into mild/dormant, moderate, or severe based on the intensity of asthma medications and occurrence of exacerbations. Ordinal logistic regression was used to estimate the probability of severity or all-cause death in the next year as a function of a patient's severity history in the past three years and selected baseline risk factors.

RESULTS: The study included 70,829 patients with incident mild asthma (62% female, mean age 31 years). Over 10 years, 8% of these patients transitioned to moderate or severe asthma. Inappropriate use of rescue medications and older age were the most influential determinants for progression from mild asthma (odds ratios: 1.79 [95% CI: 1.68, 1.90], p<0.001, 1. 24 per ten-year increase in age [95% CI: 1.22, 1.27] , p<0.001), whereas the presence of allergic rhinitis had no significant effects (OR=0.95 [95% CI: 0.91, 1.00], p=0.063).

CONCLUSION: Mild asthma remains largely stable over time. However, potentially modifiable factors such as inappropriate use of rescue medications are associated with a worsened prognosis.