The non-proportional Venn diagram explaining the components of chronic obstructive pulmonary disease (COPD) has been a graphical “COPD evergreen” along the Fletcher-Peto curve ever since it was proposed by Snider.1 The diagram has very much been intended as a way of visualising the concepts behind the processes contributing to COPD, but over the last few years there has been a growing interest in quantifying the various components of the diagram.2 3 These studies have used registered administrative diagnoses or self-reported diagnoses with the possibilities that errors in diagnostic labelling may have introduced significant bias. In this issue of Thorax Marsh et al4 present a proportional Venn diagram based on clinical data, pulmonary function measurements and CT findings from an ongoing population study in New Zealand (see page 761).

Marsh et al found that asthma was a significant cause of COPD in their general population sample; 55% of patients with COPD fulfilled their criteria for asthma whereas 30% had questionnaire-derived chronic bronchitis and 32% had emphysema defined by CT or reduced transfer factor. The overwhelming—and somewhat surprising—impact of asthma on COPD is not substantially different from that found in the previous studies;2 3 nevertheless, it challenges our traditional views and deserves attention. In addition, a significant proportion of patients with COPD were never smokers (36% overall, ranging from 13% among COPD patients with emphysema to 38% among COPD patients with asthma).

The quick …