Spirometry and ethnicity
|Fig. 1 - FEV1/FVC ratio in healthy females of different ethnic origin.|
It is well known that pulmonary function differs between ethnic groups. In the past one used “ethnic correction factors”, implying that predicted values for a pulmonary index of, for example, black subjects were calculated as being about 15% below those of whites. These “correction factors” were determined empirically in adults. The availability of a large number of spirometric records from 3-95 year old subjects of different ethnic background allowed the Global Lung Function Initiative to look into ethnic differences in greater depth. Fig. 1 illustrates an important observation: with the exception of South East Asians (southern China, Thailand, Korea), the FEV1/FVC ratio is the same in all ethnic groups at any given age and height. This implies that differences in FEV1 and FVC between ethnic groups are proportional, and independent of age. Biologically this makes sense. After all, all ethnic groups belong to the genus Homo sapiens, i.e. mammals comprising subgroups that adapted to different local conditions and differ in socio-economic backgrounds. In an evolutionary process covering millions of years mammals have been provided with a scalable lung design; as it is scalable it fits small and large animals, catering for their metabolic and other needs under widely different circumstances . Differences in pulmonary indices between ethnic groups are therefore no more than a matter of different scale. Based on this finding of proportional differences we can now add ethnic group to our model, as follows:
log(Y) = a + b•log(height) + c•log(age) + d•Ethn + spline + error
Ethnicity (Ethn) is now a co-factor. Mean differences in pulmonary function of a number of ethnic groups, relative to whites, are shown in the table. A group “Mixed/other” denotes people of mixed ethnic descent; the figures in the table are an estimate for this group, pending further studies.
|North East Asian||-0.7||-2.1||1.1||-2.7||-3.6||0.9|
|South East Asian||-13.0||-15.7||2.9||-9.7||-12.3||2.8|
The above represents an important step forward, as all ethnic groups can now be included in the regression equation. This does not solve all problems, as there appear to be differences in the scatter around predicted values. This implies that it is necessary to adjust the model for the coefficient of variation, shown earlier, as follows:
|Fig. 2 - Predicted FEV1/FVC ratio and lower limit of normal in healthy females of different ethnicity.|
log(CoV) = a + b•log(age) + d•Ethn + spline + error
The FEV1/FVC ratio is a pivotal objective index for diagnosing pathological airway obstruction. Whereas the predicted values for this ratio differ scarcely between ethnic groups, the lower limit of normal is clearly different. The GOLD group considered it too difficult to calculate the lower limit of normal for the FEV1/FVC ratio and decided that it was much easier to adopt a fixed lower limit of normal of 0.70. A lot of criticism has been published about the unscientific approach and the lack of any evidence that obstructive lung disease can thus be properly diagnosed. See for example an Open Letter, signed by a large number of reputable researchers and clinicians . The figure on the right also discloses that the GOLD criterion might lead to the spurious finding that COPD (chronic obstructive pulmonary disease) is less prevalent in East Asians, as the lower limit of normal (the 5th percentile) for FEV1/FVC remains above the 0.70 limit until a higher age than in whites and blacks.
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- Quanjer PH, Enright PL, Miller MR et al. Open Letter. The need to change the method for defining mild airway obstruction. Eur Respir J 2011; 37: 720-722. Manuscript
Acknowledgement: Figure 1 modified and reproduced with permission of the European Respiratory Society. Eur Respir J December 2012 40:1324-1343; published ahead of print June 27, 2012, doi:10.1183/09031936.00080312.