A Cross-Sectional Comparative Evaluation of the Forced Oscillation Technique and Spirometry in Patients Suspected of Having Obstructive Airway Disease at a Tertiary Care Centre

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Introduction Spirometry, the standard diagnostic tool for obstructive airway disease (OAD), is limited by effort dependency and poor sensitivity for small airways dysfunction. The forced oscillation technique (FOT) provides a non-invasive, effort-independent physiological assessment. This study aimed to evaluate the baseline physiological correlation between FOT and spirometry indices in suspected OAD and assess the role of FOT in patients unable to perform spirometry. Methods This cross-sectional observational study enrolled 201 patients presenting with clinical suspicion of OAD at a tertiary care pulmonology department. All participants underwent FOT and spirometry with post-bronchodilator assessment. Airflow obstruction and bronchodilator responsiveness on spirometry were defined as per European Respiratory Society/American Thoracic Society (ERS/ATS) guidelines. FOT abnormalities and bronchodilator responsiveness were interpreted using previously published literature-based thresholds for oscillometric parameters. Body plethysmography was additionally performed. Patients were stratified into three groups: Group A, obstruction on both spirometry and FOT, n=135 (67.2%); Group B, normal spirometry with obstruction on FOT, n=57 (28.4%); and Group C, unable to perform spirometry with obstruction on FOT, n=9 (4.5%). Statistical analysis was performed using SPSS version 24.0. Results FOT peripheral indices, particularly AX, X5, and R5-R20, showed the strongest correlations with percentage of predicted forced expiratory volume in one second (FEV₁%). Bronchodilator responsiveness was highest in Group B. FOT parameters worsened progressively with increasing obstruction severity, and percentage changes in FOT indices after bronchodilator administration correlated strongly with FEV₁% change. Conclusion FOT demonstrated excellent feasibility and identified airway obstruction in patients with preserved spirometry, supporting its complementary role in the physiological assessment of OAD. Peripheral oscillometry indices serve as valuable markers of small airways dysfunction and bronchodilator responsiveness.