Assessing small airway function for early detection of lung function impairment

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We thank F. Trinkmann and co-authors for their appreciation of our work and also in raising important questions on the role of small airway function in early detection of lung function impairment associated with asthma and smoking, leading to development of COPD 1]. The focus of our study was not simply on small airway function but rather, the onset and trajectories of lung function impairment associated with asthma and smoking. However, we agree that small airway function is an important part of that assessment. The authors raise two important points in this regard. First, that (at a microscopic level) significant pathology is present in the small airways before it can be detected by commonly used diagnostic tests, and secondly that other tests such as oscillometry, static lung volumes, transfer factor and radiological imaging add to the assessment of small airway function. We agree with both these points. However, we disagree with their assertion that assessment of small airway function using spirometry, and more specifically, forced expiratory flow at 25–75% of forced vital capacity (FEF_25–75) is neither useful nor sensitive [2].