Burney and Hooper1 recently explored the relationship between forced vital capacity (FVC) and subsequent mortality in Caucasians and African Americans aged 45–64 years from the Atherosclerosis Risk In Communities study. They found that compared with the Caucasians, the African Americans had lower age–height-adjusted FVC and higher age-adjusted mortality. Neither of these observations is novel (http://www.cdc.gov/nchs/data/databriefs/db64.pdf.)—indeed the first has led to the development of ethnic-specific spirometry norms such as the Global Lung Function Initiative equations.2
Burney and Hooper then predicted mortality in individuals, adjusting for FVC, ethnic group, age and other covariates. They developed three distinct models, using (i) raw FVC, (ii) percent predicted FVC (FVC%) based on NHANES reference equations for Caucasians, and (iii) FVC% based on NHANES ethnic-specific equations. The results for models (i) and (ii) showed no ethnic group differences in mortality, whereas model (iii) showed highly significantly greater mortality in the African Americans. These results led the authors to conclude that the use of ethnic-specific ventilatory function reference equations is inappropriate for assessing prognosis.
This is a neat conclusion, but unfortunately it is almost certainly wrong. The article makes the classic epidemiological error of assuming that association is the same as causation. The authors provide no evidence to justify their claim that the lower FVC in African Americans ‘caused’ their raised mortality, as opposed to the alternative interpretation that the association arose through other factors.
It would be interesting, for example, to repeat the mortality analyses in Table 3 using the FEV1/FVC ratio, another spirometry index known to be highly predictive of mortality3 and yet which is virtually independent of ethnic background.2 This would contradict the conclusion drawn for FVC, as the FEV1/FVC ratio was higher, not lower, in the African Americans in the current study1 and hence cannot be used to argue away the higher mortality.
Although the models are adjusted for confounding factors, the demographic and social differences between the two ethnic groups are substantial (including much higher smoking rates in the African Americans), and the results are therefore likely influenced by residual confounding. Investigations of ethnic differences in the relationship between FVC and mortality should preferably exclude all smokers and match socio-economic backgrounds much more closely than in the present study.
Ethnic differences in lung function have long been investigated and debated and, to our knowledge, no single study has been able to attribute all differences to body size, nor to socio-economic factors. The ethnic differences in lung function between African Americans and Caucasians are likely due to a combination of physiological, social and environmental influences. Given obvious ethnic variation in body physique even among well-nourished healthy subjects, it is highly unlikely that a crude measure of standing height can adequately explain ethnic variation in thoracic and hence lung volume. Failure to take such differences into account risks significant over-diagnosis of lung disease amongst the non-White community.
Conflict of interest:All authors have been involved in the Global Lung Function Initiative to develop multi-ethnic all-age spirometry reference equations.