Day-to-day variability of oscillatory impedance and spirometry in asthma and COPD

Variability in airway function may be a marker of disease activity in COPD and asthma. The aim was to determine the effects of repeatability and airway obstruction on day-to-day variability in respiratory system resistance (Rrs) and reactance (Xrs) measured by forced oscillation technique (FOT). Three groups of 10 subjects; normals, stable asthmatic and stable COPD subjects underwent daily FOT recordings for 7 days. Mean total and inspiratory Rrs and Xrs, and expiratory flow limitation (EFL) Index (inspiratory – expiratory Xrs), were calculated. The ICC's were high for all parameters in all groups. Repeatability, in terms of absolute units, correlated with airway obstruction and was therefore lowest in COPD. Day-to-day variability was due mostly to repeatability, with a small contribution from the mean value for some parameters. FOT measures are highly repeatable in health, stable asthma and COPD in relation to the wide range of measures between subjects. For home monitoring in asthma and COPD, either the coefficient of variation or individualized SDs could be used to define day-to-day variability.

► Impedance measured by FOT is highly repeatable in health, asthma, and COPD.
► Repeatability of total and inspiratory components of Rrs and Xrs in absolute terms, reduces with increasing airflow obstruction.
► Day-to-day variability is determined by airflow obstruction and repeatability.
► Knowledge of this variability is essential when planning home monitoring studies, especially in COPD
► The results suggest it is important to establish an individual's variability in a stable state prior to commencement of a prospective study.