• COPD epidemiology

We thank Dr Miller for his comments1 on our recent article.2 While we agree that age is an important factor in this longstanding controversy, we would like to debate some of the points he makes. First, contrary to his assertions, strong relationships have been shown between CT measures of emphysema and pathological findings.3 While there has, in the past, been discussion regarding the ‘best’ Hounsfield Unit (HU) threshold to define CT emphysema, our use of −950 HU, correlating with more severe disease, has been used widely to demonstrate the power of CT-based density metrics in identifying COPD phenotypes, monitoring longitudinal changes and reducing the population size needed to power pharmaceutical studies.4 ,5 Second, while we agree with Dr Miller that age may impact the CT-based emphysema score, the effect size is considerably smaller than suggested. Recent data from COPDGene examining normal subjects with a similar age range as our study, with identical scanner protocols, was unable to detect a significant age effect.6 Analyses of data from the Multi-Ethnic Study of Atherosclerosis Lung study, which included a much larger and older population, (n=854) showed a small age effect (0.2% for every 10 years) on CT-emphysema.7 In another large multicentre study, Coxson et al 8 noted that the progression of CT-emphysema with advancing age is low. …