Impulse oscillometry reference values and small airway reversibility in the assessment of asthma.

BACKGROUND: The currently recommended bronchodilator reversibility (BDR) criteria have relatively low diagnostic sensitivity in asthma. Reference values and small airway reversibility (SAR) determined by impulse oscillometry (IOS) have been reported, but how they should be applied in the assessment of asthma remains unclear. OBJECTIVE: To generate IOS reference values in healthy adults and to evaluate the diagnostic performance of baseline IOS parameters and SAR in treatment-naïve asthma. METHODS: A total of 289 healthy controls and 290 treatment-naïve asthma patients were enrolled. Sex-specific reference equations for IOS-derived Z-scores were generated from the healthy cohort. The diagnostic performance of baseline IOS parameters and SAR indices for discrimination between asthma and healthy controls was evaluated. RESULTS: Reference-defined small airway dysfunction (SAD) was present in 15.2% to 24.5% of asthmatic patients versus 3.1% to 6.6% of healthy controls across the three IOS parameters (R5-R20, X5 and AX). ROC-derived cutoffs lay within the reference range. Among baseline values, R5-R20 discriminated best (AUROC 0.74; cutoff > 0.08 kPa/L/s, sensitivity 66.9%, specificity 68.5%), followed by AX (0.69) and X5 (0.63). A positive SAR criterion, defined as a significant change in any IOS parameter, achieved a diagnostic sensitivity of 77.8%, substantially higher than conventional spirometric BDR (24.5%). Among patients with preserved pulmonary function (PPF) and absent BDR, SAR maintained a sensitivity of 71.5%. CONCLUSION: IOS reference values characterize SAD rather than identify asthma. ROC-derived SAR thresholds may complement spirometry by improving sensitivity, particularly in patients with PPF and absent BDR.