Expression of bronchodilator response using forced oscillation technique measurements: absolute versus relative.

We refer to the recent study by OOSTVEEN et al. [1] on bronchodilator response in 4-yr-old children with different wheezing phenotypes. The forced oscillation technique is particularly suited to 2–7-yr-old children, and its suitability to assess bronchodilator responsiveness is still being investigated. Thus, the study is of great clinical relevance and, though of a narrow age range, is a welcome step forward from previous studies characterising bronchodilator responses assessed using measures of airway resistance in preschool children [2–4]. However, we would like to raise a point we feel to be of importance. The authors argued for the case of using the absolute change pre-and post-bronchodilator to express a bronchodilator response in children assessed using forced oscillation technique (FOT) variables. They reasoned that expressing the response as a relative change from baseline homogenises the different responses of subjects with different baseline bronchomotor tone, reflected by their different baseline lung function values. The reason we advocate the use of the relative over the absolute response assessed using airway resistance from oscillometric methods is that the absolute response is generally dependent on lung function at baseline [2, 4]. Statistically, assessment of magnitude of a treatment effect should be adjusted for any differences between the groups or variability in the pretreat-ment values [5]. For instance, for the forced expiratory volume in 1 s (FEV1) it has been pointed out that since the absolute change between repeated measurements is constant across patients with varying FEV1 levels, it is the absolute change that should be used when expressing any treatment differences [6]. More accurately, expressing the bronchodilator response using the absolute change is valid provided this change is independent of the baseline value, otherwise the relative change should be used [7]. Dependence on age or height can be accounted for using % predicted values. Note that expressing the bronchodi-lator response in terms of absolute changes in z-scores is statistically identical to using absolute changes when the data are homoscedastic, though it may still be useful to use z-scores when lung function is already expressed in this manner or if the data are heteroscedastic. In more physiological terms, for airway resistance measures, the smaller the airway calibre (hence the larger the baseline resistance), the more profound the effect of any increases to airway calibre. Thus, the same change in airway calibre represents a larger effective change in a small airway than in a large airway. Bronchomotor tone …