Background: Achieving and maintaining symptom control is a primary goal of asthma management. Although factors associated with the likelihood of achieving asthma control have been studied, there are unanswered questions as to why subjects remain at a given asthma control level, which we term the stability of asthma control.

Methods: We evaluated the stability of asthma control using a longitudinal cohort study of mild–moderate asthmatics. Participants underwent 5 assessments during the 1-year follow-up period. We a) evaluated variability in the stability of symptom control between individuals, and b) used a random-effects logistic regression model to evaluate the impact of a suite of factors on the stability of asthma control.

Results: 431 individuals (67% female, mean age 51.6) contributed 1,717 study visits. Individuals varied from completely stable in their level of asthma control during the entire study period (18% of participants) to completely unstable (12%), the latter meaning that control level changed between each subsequent visit. Comorbidity was negatively associated with the likelihood of changing control levels (OR 0.94, 95% CI 0.89–0.99, p=0.01). No other exposure was associated with a change in asthma control. Only 4% of between-individual variation in the stability of asthma control was explained by the exposures included in our model.

Conclusions: The tendency to remain at a given control level varies significantly between asthma patients. Only a small fraction of this variability is explained by observable characteristics. Heterogeneity in the stability of asthma control should be considered when monitoring asthma control in patients.