The FEV1 and FEV1/FVC ratio seems to have become the predominant, if not the sole factor for determining the presence of airway obstruction. It is true that a reduced FEV1/FVC ratio provides a strong and reliable signal for this purpose but its limitations have also been recognized for quite a while. The most obvious one is that the FEV1/FVC ratio will be falsely elevated when the FVC is underestimated. This is the primary factor driving the interest in FEV6 and the FEV1/FEV6 ratio. Less well appreciated is the fact that there are many causes and sites within the airways that can be involved in airway obstruction and that the focus on the FEV1/FVC ratio may cause certain forms of airway obstruction to be overlooked.
The FEF25-75 (aka MMEF) was originally proposed as way to determine the presence of small airways disease but it has since been shown to be an unreliable indicator. Most of the pulmonary physicians I work with have expressed doubt that there is such a thing as small airways disease but that doesn’t mean that some patients don’t have mild airway obstruction that is not evident when assessed solely by the FEV1/FVC ratio.