Comparative evaluation of impulse oscillometry and spirometry for the detection of small airway disease in patients with obstructive airway diseases

Although there is some evidence supporting a correlation between impulse oscillometry (IOS) and spirometry for detecting small airway disease (SAD), in many cases, discordant results are observed. IOS has not yet been established as a standard method for routine pulmonary function assessment in clinical practice. We did this study with the primary objective of diagnosing SAD in patients with obstructive airway diseases (OAD) using IOS and comparing it with spirometry. This cross-sectional, comparative study was conducted from July 2023 to December 2024. Adult patients (≥18 years of age) with diagnosed stable OADs [asthma, chronic obstructive pulmonary disease (COPD), and bronchiectasis] were included in the study (n=118). Maximum expiratory flow 25-75 with a Z score of <-1.64 and resistance at 5 Hz - resistance at 20 Hz (R5-R20) >150% were criteria used for defining small airway obstruction in spirometry and IOS, respectively, and were compared. As secondary outcomes, correlation of severity of SAD across COPD groups, Global Initiative for Asthma (GINA) steps using IOS, and comparison of bronchodilatation responsiveness (BDR) were also done. In pre-BDR, IOS and spirometry diagnosed SAD in 94.91% and 82.20% of patients, respectively (p=0.069). There was poor agreement (ĸ=0.155) between these modalities pre-BDR; however, post-BDR, there was fair agreement (ĸ=0.277) between these modalities, and both modalities for the detection of SAD were significantly related (p=0.003). Across various Global Initiative for Chronic Obstructive Lung Disease groups, there was no significant difference in the severity of SAD, while across various GINA steps, there was a significant difference in the severity of SAD, especially GINA step 1-2 and step 5. There was poor agreement between these modalities for the BDR test. In conclusion, both modalities performed differently in pre-BDR (discordant) and post-BDR (concordant) settings. For the BDR test, both modalities are not exchangeable. Further research is warranted to determine the optimal setting (pre-BDR vs. post-BDR).