Respiratory epidemiology Figure 6 Percentage change in forced vital capacity (FVC) relative to predicted FVC, after bronchodilator (BDR FVCp) 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 9%. We observed signiﬁcant heterogeneity across sites (p<0.0001), with 80% of the overall variability in the data attributable to siteesite variability. there was a short-term intra-subject variability of 200 ml in ﬂow volume indices.34 Although the long-term variability in lung function in patients is larger than in healthy subjects, the short- term variability in patients in a stable condition is very similar to normal subjects, with an observed 95% CI of change in FEV1 of 190 ml and 12.3%.15 16 This could be a potential explanation for the similarity between threshold values in our study and the guideline recommendations.10 There is no consensus on how BDR is best expressed: in absolute terms or as percentage of the initial value,10 12 or as percentage of predicted value.13 Of the six indices of BDR in this global study, we showed that the percentage change in FEV1 relative to either baseline or predicted were the most stable across sites, while there was signiﬁcant site-to-site variability in all FVC estimates and in absolute FEV1 change. Overall, this population-based evidence supports the use of 10% increase relative to predicted FEV1 to determine bronchodilator revers- ibility, as this feature is stable across sites, is independent of sex and stable across GOLD stages, yet discriminates healthy subjects from obstructive individuals. Nevertheless, in practice, it would be prudent to couple any percentage threshold with a qualifying volume threshold.10 This is the ﬁrst study to compare BDR in population-based samples from a large number of sites on several continents. This information adds to the growing body of data on lung function that will help us to understand the range of normality and what may inﬂuence this. Other strengths of the study include the careful attention to standardisation and quality control used across all sites in the BOLD study21 22 and the well recognised technique of deﬁning the 95th percentile in the healthy popu- lation for deriving the reference for BDR thresholds.18 19 In addition, the analysis of FVC data using different criteria is much more comprehensive than in other reports.6 A potential limitation of the study is that our deﬁnition of healthy never smokers relies on self-report of doctor-diagnosed lung disorders rather than objectively deﬁned obstructive lung The proportion of people in excess of thresholds for overall cohort, for subgroups of normal, GOLD stage 1, GOLD stage 2 and GOLD stage Overall N1¼10 217 6.7% 7.8% 6.0% N2¼9546 7.5% 7.5% 6.8% Normal N1¼7931 5.6% 4.4% 4.3% N2¼7697 4.0% 3.8% 3.2% GOLD stage 1 GOLD stage 2 GOLD stage 3e4 N1¼846 11.5% 11.6% 12.3% N2¼819 21.2% 17.0% 21.6% N1¼790 12.2% 27.3% 13.9% N2¼764 20.7% 24.7% 20.5% N1¼219 7.3% 43.4% 9.1% N2¼212 29.7% 42.5% 27.8% Table 4 3e4 Variables FEV1-based thresholds DFEV1 (>284 ml) DFEV1i (>12%) DFEV1p (>10%) FVC-based thresholds DFVC (>322 ml) DFVCi (>10.5%) DFVCp (>9.2%) DFEV1 (ml) and DFVC (ml) are absolute change after bronchodilator; DFEV1i and DFVCi are change as a per cent of initial value; DFEV1p and DFVCp are change as per cent of predicted value. N1 is number with non-missing FEV1 values and N2 is number with non-missing FVC values. Normal ¼ not classiﬁed as any GOLD stage; GOLD stage 1 ¼ FEV1/FVC <0.7 and FEV1 % predicted >80%; GOLD stage 2 ¼ FEV1/FVC <0.7 and FEV1 % predicted <80% and >50%; GOLD stage 3e4 ¼ FEV1/FVC <0.7 and FEV1 % predicted <50%. COPD, chronic obstructive pulmonary disease; FEV1, forced expiratory volume in 1 s; FVC, forced vital capacity; GOLD, Global Initiative for Chronic Obstructive Lung Disease. 724 Thorax 2012;67:718e726. doi:10.1136/thoraxjnl-2011-201445