Comparison of oscillometry ratios between IOS and AOS in patients with asthma and COPD

The forced oscillation technique (FOT) enables effort-independent assessment of small airway dysfunction (SAD) in obstructive lung diseases. We compared resistance and reactance ratios between two FOT modalities—impulse oscillometry (IOS) and airwave oscillometry (AOS), in patients with asthma and COPD. We retrospectively analysed paired pre- and post-bronchodilator IOS and AOS measurements from 82 patients (58 asthma, 24 COPD). Resistance (R5–R20/R5 for IOS; R5–R19/R5 for AOS) and reactance (X5/AX) ratios were compared using correlation and Bland-Altman analyses. Resistance ratios showed significant agreement between devices, with minimal mean differences. Reactance ratios were also significantly correlated but were higher with IOS than AOS. This difference increased at higher X5/AX ratios, likely due to device specific signal characteristics and calibration differences. Post-bronchodilator improvements were observed with both devices, with a greater change in X5/AX using IOS (p < 0.05). In conclusion resistance ratios were comparable between IOS and AOS, supporting their clinical use across devices. In contrast, reactance ratios differed, highlighting the need for device-specific normative values and improved standardization. FOT-derived ratios may offer a practical alternative to absolute values for assessing SAD in asthma and COPD.