Chronic airway diseases are obstructive lung disorders occurring more and more frequently, therefore becoming a major public health burden worldwide [
1]. Severity assessment and management of these diseases are defined according to clinical examination and routine pulmonary function tests (PFTs). However, these measurements are not strong enough to accurately discriminate patients regarding clinical outcomes [
2]. Chronic obstructive pulmonary disease (COPD) and asthma are the most common causes of such chronic airflow limitation. From a pathophysiological point of view, small airways (defined from 2 mm in internal diameter and downward) are regarded as the underlying elective anatomic level of airway obstruction for both disorders [
3,
4], but are inadequately investigated using conventional PFTs, namely “the lung’s quiet zone” [
5]. Specific structural changes at this level can be assessed using tools such as the single-breath or multiple-breath nitrogen washout test (SBNT, MBNT respectively) [
6]. SBNT and MBNT are time consuming and the routine uses are not widely available owing to limited access to the equipment. Due to the fact that expiratory acquisition allows indirect evaluation of bronchiolar involvement [
7], computed tomography (CT) has been increasingly put forward as an appropriate non-invasive tool for refinement in the classification and treatment monitoring of COPD and asthma [
8‐
14]. The application of an increasingly wide range of technological tools also allows post-processing by segmentation software and thus quantifies air trapping objectively. Both a decrease in mean lung density and the percentage of low attenuation area on expiratory CT have been used in various study and correlated with disease severity in COPD and asthma [
15‐
17]. Some authors have also suggested the role of paired expiratory to inspiratory ratio or difference as a small airway marker [
18‐
20]. However, the choice of one of these criteria may affect the results and there is, therefore, a crucial need for standardization in CT air trapping expression.
This prospective study was designed firstly to explore the validity of air trapping software-assisted CT quantification for the assessment of small airway obstruction by using SBNT as standard of reference and, secondly, to assess which was the most accurate criterion.