Respiratory epidemiology Figure 4 Absolute change in forced vital capacity (FVC) after bronchodilator (BDR FVC, ml) expressed as 95th percentile with 95% CI in healthy non- smokers. Those sites whose estimates are most stable get greater weight (right-hand column) in constructing the pooled estimate of 322 ml. We observed signiﬁcant heterogeneity across sites (p<0.0001), with 75% of the overall variability in the data attributable to siteesite variability. thresholds were included as the ATS-ERS recommendation included these10 to accommodate ﬂow responders and volume responders to inhaled bronchodilators.32 33 However, it is worth noting that the issue of heterogeneity is much more a concern for FVC than for FEV1, at least so long as the latter is expressed as a per cent change of initial or predicted value. Comparison of our healthy population-based thresholds with the thresholds of guidelines based on patient studies provided insight into the short-term variability of BDR measurements. In this global study, the 95th percentile cutoffs of BDR in healthy never smokers of 12% for DFEV1i was identical to the cutoffs of 12% change in FEV1 or FVC for a positive response required by international guidelines,10 while 10% for DFEV1p was similar to the 9% change found by one study.8 The guideline recommen- dation for a positive BDR had been based on observations in selected patients with asthma and COPD, and on the fact that Figure 5 Percentage change in forced vital capacity (FVC) relative to initial baseline, after bronchodilator (BDR FVC1i) expressed as 95th percentile with 95% CI in healthy non-smokers. Those sites whose estimates are most stable get greater weight (right-hand column) in constructing the pooled estimate of 11%. We observed signiﬁcant heterogeneity across sites (p<0.0001), with 66% of the overall variability in the data attributable to siteesite variability. Thorax 2012;67:718e726. doi:10.1136/thoraxjnl-2011-201445 723