Respiratory epidemiology Figure 2 Percentage change in forced expiratory volume in 1 s (FEV1) relative to initial baseline, after bronchodilator (BDR FEV1i) expressed as 95th percentile with 95% CI in healthy non- smokers. The pooled estimate is 12% with no evidence of cross-site heterogeneity. from this study provided new data relevant to this gap in information. The ‘cutoff ’ values in our analysis differ somewhat compared with those from single site population studies of healthy subjects. For deﬁning a positive response, these studies had suggested 95th percentile thresholds of 130 ml18 or 240 ml19 for absolute change in FEV1; 9%18 or 5.9%19 for percentage change in FEV1 from initial value (%ΔFEV1i) and 9%8 18 for change in FEV1 relative to predicted value (%ΔFEV1p). In our study, the derived cutoffs for %ΔFEV1i and %ΔFEV1p are higher but stable as evidenced by the lack of site heterogeneity. Also, these threshold values complied with the caution to exceed the measurement variability of at least 8% change within individ- uals for deﬁning an unambiguous response.8 30 31 The FVC Figure 3 Percentage change in forced expiratory volume in 1 s (FEV1) relative to predicted FEV1, after bronchodilator (%DFEV1p) expressed as 95th percentile with 95% CI in healthy non-smokers. The pooled estimate is 10% with no evidence of cross-site heterogeneity. 722 Thorax 2012;67:718e726. doi:10.1136/thoraxjnl-2011-201445