Discriminating Measures and Normal Values for Expiratory Obstruction: Conclusion
In evaluating the cigarette-smoking effect in the NHANES III population, both the decreases in FEV1/FVC and increases in 1 β FEV3/FVC for a given age group are striking (Fig 3). Using either measurement, fractions progressively deteriorate with age relative to the never-smoking subjects. Consequently, by middle age current smokers have similar values to those of never-smokers who are about 20 years older (Fig 3). In both our study (Table 3) and that of Miller et al, abnormal FEV3/FVC values were as common as abnormal FEV1/FVC values. This suggests that attention similar to that accorded to FEV1/FVC should be paid to FEV3/FVC and 1 β FEV3/FVC in evaluating airway obstruction. Because values for both FEV1/FVC and FEV3/FVC normally decline at a relatively constant rate with aging, using fixed ratios such as 70% or 73% for FEV1/FVC or any other value for FEV3/FVC as the mean or LLN values should be avoided. www.canadian-familypharmacy.com
Measurements (in liters per second) of either instantaneous flow (ie, forced expiratory flows of 25%, 50%, 75%, or 85% of FVC) or average flow (FEF25-75 or FEF75-85) over any given volume are necessarily dependent on both flow and volume measurements. As Miller et alβ point out, such flow measurements at differing volumes are inherently and necessarily variable. The inherently high variability of FEF25-75, in both never-smoker and current smoker groups, is confirmed in Figure 2. The error of using 80% of the mean predicted value has been confirmed and emphasized by many authorities, and has been reconfirmed by our finding that approximately 25% of healthy never-smoking NHANES III adults have FEF25-75 values < 80% of the mean predicted value.
Even when 95% confidence limits for FEF25-75 are used, there are an inordinate number of individuals with probable false-negative results (42%) who clearly have obstructive airways disease, especially among those who are > 60 years of age (Table 3). Probable false-negative results for FEF25-75 measurements are also frequent in subjects with abnormally low FEV3/FVC values (ie, those with later expiratory obstruction or obstruction of long-time-constant airways). On the other hand, apparent false-positive FEF25-75 values (Table 3) are found almost exclusively in younger adults. The fact that the LLN for FEF25-75 is an absolute rather than a relative value in all predicting equations is a factor in the high incidence of probable false-negative results.