Table 3 The prevalence (as a percentage) of airway obstruction in three large population based samples using three reference equations and different definitions of airway obstruction applied to each population

Age	n	FEV₁/FVC <0.70	FEV₁/FVC <LLN			FEV₁/FVC <LLN and FEV₁<80%pred
Age	n	GOLD	NHANES III	HSE	NL	NHANES III	HSE	NL
Dutch
<50	2965	12.2	17.2	20.7	11.1	4.6	5.2	4.4
⩾50	1592	36.1	26.6	29.3	14.6	15.5	14.9	9.9
All	4557	20.5	20.4	23.7	12.3	8.4	8.6	6.3
English
<50	14 506	6.0	9.2	11.8	6.7	2.4	2.4	2.6
⩾50	10 098	24.0	15.7	17.3	9.1	14.2	13.5	6.7
All	24 604	13.4	11.9	14.1	7.7	7.2	6.9	4.3
American
<50	3196	6.3	10.2	12.4	6.9	2.7	2.8	4.4
⩾50	3633	35.5	20.2	21.4	10.2	16.8	15.1	7.8
All	6829	21.8	15.5	17.2	8.7	10.2	9.4	5.8

Different definitions of airway obstruction include: the GOLD criterion; FEV₁/FVC below the LLN for three reference equations; and the last three columns apply the FEV₁% pred <80% (BTS and NICE) criteria together with the FEV₁/FVC <LLN criteria for each reference equation.
The most appropriate reference equations are internal to the study (indicated by the bold numbers). Analysis in Americans was limited to Caucasian subjects. Reference equation sources: NHANES III26 and HSE.20 NL is the Dutch reference equations (see appendix in the online data supplement).
BTS, British Thoracic Society; FEV₁/FVC, forced expiratory volume in 1 s/forced vital capacity; GOLD, Global Initiative for Chronic Obstructive Lung Disease; HSE, Health Survey for England; LLN, lower limit of normal; NICE, National Institute for Health and Clinical Excellence; NHANES III, National Health and Nutrition Examination Survey III.