# Association between impulse oscillometry Z-scores and asthma control and exacerbation risk in a tertiary severe asthma clinic

## TL;DR

This study finds that abnormal oscillometry Z-scores in asthma patients are linked to worse asthma control and higher risk of asthma attacks.

## Contribution

The study introduces Z-scores from impulse oscillometry as a novel predictor of asthma control and exacerbation risk in severe asthma patients.

## Key findings

- Elevated R5 Z-scores are associated with increased risk of asthma exacerbations.
- Z-scores for R5, X5, and AX correlate with asthma control levels even after adjusting for other factors.
- Higher Z-score severity thresholds predict poorer asthma control and exacerbation risk.

## Abstract

Respiratory oscillometry is a sensitive tool for assessing small airways dysfunction. However, limited evidence on cutoff values for interpretation remains a barrier to its clinical use. The aim of this study was to determine whether the presence and severity of abnormalities, defined by Z-scores for oscillometric parameters, are associated with asthma symptoms and exacerbation risk.

We retrospectively reviewed the medical records of all patients with asthma managed in a severe asthma clinic between 2019 and 2022 who underwent routine oscillometry. Z-scores for oscillometric parameters were analyzed as continuous and categorical variables to assess their associations with asthma control and exacerbation risk.

When analyzed as categorical variables, Z-score-defined severity thresholds for resistance (R5), reactance (X5), and the area under the reactance curve (AX) were associated with levels of asthma control (as measured by the ACQ5). When analyzed as continuous variables, Z-scores were also correlated with worst asthma control (as assessed by both ACQ5 and the asthma control test) (P < 0.005). These correlations remained significant after adjustment for spirometric indices, FeNO, and treatment changes. Elevated Z-scores (>1.64) for R5 were associated with a higher risk of exacerbations (OR 2.70, 95% CI 1.27–5.17, P = 0.009). The risk of exacerbation increased with the severity of airway obstruction. Similar trends were observed for AX and X5; however, these associations did not reach statistical significance.

The presence and severity of airway obstruction, as defined by R5 Z-scores, predict poorer asthma control and an increased risk of exacerbations. Similar associations with asthma control were also observed for X5 and AX Z-scores. Clinicians should use Z-scores over other cutoffs to aid interpretation.