AHR and respiratory tract infectious disorder: Last but not least, the present cross-sectional study only measured the AHR levels of patients at a single time point, which makes it difficult to reflect the level of asthma control over a period of time, given that the airway conditions of asthmatic patients may vary due to different factors, like season, exposure to trigger agents, and medication use, etc. The impacts of these factors were minimised by excluding patients with recent (within four weeks) exacerbation and respiratory infections from our study.