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

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 (A_X) 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 A_X 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 A_X Z-scores. Clinicians should use Z-scores over other cutoffs to aid interpretation.