I was reading James Hansen’s textbook on pulmonary function testing and ran across a spot where he made a minor criticism of the NHANESIII (Hankinson et al) reference equations for the FEV1/FVC ratio. Specifically he noted that the NHANESIII equation ignored height and only used age as a variable but that when he compared the directly calculated FEV1/FVC ratio with one indirectly derived from predicted FEV1 and FVC there was a discrepancy across the normal ranges of height of up to 2.4%.
I had also noticed this discrepancy and wrote about it a while back but at the time I’d only been discussing my lab’s adoption of the NHANESIII reference equations. Hansen’s observation intrigued me, so I decided to re-visit this issue more systematically.
To do this I’ve taken 23 different reference equations for men and women and a variety of ethnicities and plotted the change in the FEV1/FVC ratio versus height, and then repeated this across a range of ages.
When I did this I saw that almost universally the FEV1/FVC ratio decreases with increasing height and that the magnitude of the decrease grew smaller with increasing age.
For men at age 20, the average predicted FEV1/FVC ratio for a height of 165 cm was 86.89 and for a height of 185 cm was 84.36, a difference of 2.53 percent. At age 65, the average predicted FEV1/FVC ratio for a height of 165 cm was 78.05 and for a height of 185 cm was 77.15, a difference of 0.90 percent. For women at age 20 the average predicted FEV1/FVC ratio for a height of 150 cm was 86.97 and for a height of 170 cm was 84.94, a difference of 2.03 percent. At age 65m the average predicted FEV1/FVC ratio for a height of 150 cm was 79.26 and for a height of 170 cm was 78.68, a difference of 0.58 percent.
This matters because an FEV1/FVC ratio below the lower limit of normal (LLN) is an indication of airway obstruction. The NHANESIII LLN for FEV1/FVC ratio does not take height into consideration and means that a tall individual is more likely to be considered to be below the LLN and a short individual is more likely to remain above the LLN.
I have a great deal of respect for the NHANESIII study since it includes an exceptionally large and ethnically diverse study group and a sophisticated statistical analysis was performed on the results. Their reason for excluding height as a factor for the FEV1/FVC ratio reference equation was not discussed however, other than stating that “Only age is needed…”. I have tended to accept this because my lab has seen some exceptionally short and exceptionally tall individuals and the predicted FEV1/FVC ratio derived from their predicted FEV1 and predicted FVC appeared to be in error.
| Male 48” | Observed: | Predicted (M) | %Pred (M): | Predicted (N) | %Pred (N) |
| FVC: | 1.72 | 1.69 | 102% | 2.01 | 86% |
| FEV1: | 1.35 | 1.68 | 82% | 1.66 | 81% |
| FEV1/FVC Ratio: | 78.6 | 97.5 | 81% | 78.4 | 100% |
| Male 84” | Observed: | Predicted (M) | %Pred (M): | Predicted (N) | %Pred (N) |
| FVC: | 8.24 | 7.40 | 111% | 8.01 | 103% |
| FEV1: | 5.41 | 5.44 | 97% | 6.36 | 85% |
| FEV1/FVC Ratio: | 65.5 | 73.5 | 89% | 81.45 | 80% |
(M) = Morris 1971 (N) = NHANESIII
These examples are from several years ago when my lab was still using Morris’ 1971 reference equations and the problem was more obvious for the shorter of these two individuals but in either case it was apparent there was a problem. Our response at the time was to use the predicted NHANESIII FEV1/FVC ratio in place of Morris’ predicted when the results were interpreted.
A good part of the problem probably lies with the Morris reference equations for individuals that are outliers in height. When the predicted FEV1/FVC ratio is calculated from the NHANESIII predicted FEV1 and FVC it is 82.6 for the shorter individual and 79.4 for the taller. Even so, the difference between the directly and indirectly calculated NHANESIII FEV1/FVC ratios are 4.60% and 2.05%.
I have been unable to find any study that has directly addressed the issue of height versus FEV1/FVC ratio. Given that 44 out of 46 reference equations show a decrease in the FEV1/FVC ratio with increasing height I have to think that this relationship is correct. My only thoughts about why this is the case would be that airway dimensions do not scale exactly with lung volume. It is still pretty amazing that the FEV1/FVC ratios are almost exactly the same over a very wide range of lung volumes but I don’t think we should be surprised that there is a difference.
Based on this observation, I think this means that the FEV1/FVC ratio should be calculated from predicted FEV1 and FVC. I’d prefer to continue to use the NHANESIII set of reference equations but there is no LLN for an indirectly calculated FEV1/FVC ratio. The LLNs for the directly calculated FEV1/FVC ratio range from 88.3% to 89.9% (based on gender and ethnicity) of the predicted FEV1/FVC ratio. I think that these would probably make a good estimate of the LLN for an indirectly calculated FEV1/FVC ratio.
| Caucasian: | Black: | Hispanic: | |
| Male: | 89.0% | 88.3% | 89.9% |
| Female: | 89.25% | 88.35% | 89.9% |
When reference equations from a large number of studies are compared a relationship between height and the FEV1/FVC ratio becomes evident. The NHANESIII reference equation for the FEV1/FVC ratio does not use height as a factor. Because of this, taller individuals are more likely to be classified with airways obstruction than shorter ones and this is likely because airway dimensions and lung volumes scale at different rates. When the NHANESIII reference equations are used the best solution would be to derive the FEV1/FVC ratio from the predicted FEV1 and FVC instead of using the reference equation for the FEV1/FVC ratio.
References:
Hansen JE. Pulmonary Function Testing & Interpretation. Jaypee Brothers Medical Publishers, 2011.
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