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

The currently recommended bronchodilator reversibility (BDR) criteria have relatively low diagnostic sensitivity for 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.

To generate IOS reference values in healthy adults and to evaluate the diagnostic performance of baseline IOS parameters and SAR in treatment-naive asthma.

A total of 289 healthy controls and 290 patients with treatment-naive asthma 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 discriminating between asthma and healthy controls was evaluated.

Reference-defined small airway dysfunction was present in 15.2% to 24.5% of patients with asthma vs 3.1% to 6.6% of healthy controls across the 3 IOS parameters (the difference between resistance at 5 and 20 Hz, reactance at 5 Hz, and area under the reactance curve between 5 Hz and resonant frequency). Receiver operating characteristic-derived cutoffs lay within the reference range. Among baseline values, the difference between resistance at 5 and 20 Hz discriminated best (area under the receiver operating characteristic curve [AUROC] 0.74; cutoff > 0.08 kPa/L/s; sensitivity, 66.9%; and specificity, 68.5%), followed by area under the reactance curve between 5 Hz and resonant frequency (AUROC 0.69) and reactance at 5 Hz (AUROC 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 that of conventional spirometric BDR (24.5%). Among patients with preserved pulmonary function and absent BDR, SAR maintained a sensitivity of 71.5%.

IOS reference values characterize small airway dysfunction rather than identify asthma. Receiver operating characteristic-derived SAR thresholds may complement spirometry by improving sensitivity, particularly in patients with preserved pulmonary function and an absent BDR.