Diagnostic Performance of Oscillometry-derived Ratios for Identifying Small Airway Obstruction: A Post Hoc Analysis

Background: Lung oscillometry is a noninvasive, effort-independent alternative to spirometry, valuable in patients unable to perform forced expiratory manoeuvres. This post hoc study assessed the diagnostic utility of oscillometry-derived ratios – resistance ratio (R5–R19/R5) and reactance ratio (X5/Ax) – for detecting small airway obstruction in clinically diagnosed asthma patients. Methods: Oscillometry data from 100 adult asthma patients unable to perform spirometry were retrospectively analyzed. R5–R19 ≥1.0 cmH₂O/L/s defined small airway obstruction. Resistance and reactance ratios were calculated. Correlations, group comparisons, and regression analyses were performed using Python 3.11 to assess the association of ratios with R5–R19 and the impact of age, sex, and obesity. Results: The resistance ratio showed a strong correlation with R5–R19 ( r = 0.857) and 100% sensitivity. The reactance ratio showed a modest correlation ( r = −0.327) with 97.1% sensitivity. Both ratios significantly differed between groups ( P < 0.00001). Regression showed the resistance ratio was unaffected by age, sex, or obesity, unlike the reactance ratio, which was influenced by sex. Conclusions: The resistance ratio (≥19%) is a reliable marker of small airway obstruction. Reactance ratio may require cautious interpretation.