Respiratory Reactance Reflects Chronic Obstructive Pulmonary Disease Burden Independent of Computed Tomography-defined Structural Changes

Background Spirometry remains the cornerstone for the diagnosis and staging of chronic obstructive pulmonary disease (COPD), but has limitations, particularly in older or frail patients. Respiratory oscillometry offers an effort-independent assessment of small-airway dysfunction; however, its clinical role and interpretability remain uncertain. Methods This retrospective study included 158 patients with COPD who had concurrent data from paired oscillometry, spirometry, and chest computed tomography (CT). The association between oscillometry parameters and a forced expiratory volume in 1 s (FEV₁) of less than 50% predicted was evaluated using logistic regression and receiver operating characteristic analyses. Emphysema severity was assessed visually using chest CT. Patients were stratified using the optimal oscillometry cut-off to examine the association between symptom burden and COPD prognosis. Results R ₅ –R ₂₀ and area under reactance curve (AX) were independently associated with the predicted FEV₁ < 50%. AX demonstrated superior predictive performance (area under the ROC curve [AUC]; 0.843) compared to R ₅ –R ₂₀ (AUC; 0.802) and was comparable to the combined model incorporating oscillometry and emphysema severity (AUC; 0.862). An AX threshold > 1.5 kPa·L⁻¹, derived from the Youden index, showed good discriminative ability, with a sensitivity of 72.7% and specificity of 83.9%. Patients with AX > 1.5 experienced a significantly greater clinical burden than those in the lower AX group. Conclusion AX is a clinically significant marker of airflow limitation and disease burden in patients with COPD. A simple threshold of AX > 1.5 kPa·L⁻¹ may aid in risk stratification, particularly if spirometry or imaging is limited.