Oddly enough, I recently got a couple emails on the same day about the FEF25-75 and ended up corresponding for a while with the authors. FEF25-75 is a subject that somehow manages to keep resurrecting itself no matter how many stakes have gotten hammered into its heart. My opinion (expressed previously), and those of many others, is that the measurement of FEF25-75 is overly affected by FVC volume and expiratory time; that its reproducibility is poor; that its “normal” range is too broad to be meaningful; and that the FEF25-75 is usually only abnormal when the FEV1 is also below the LLN. Despite all this the FEF25-75 still continues to be used by many clinicians and researchers.
While discussing it however, one of the points that came up was how the “best” FEF25-75 should be selected. Given that it’s not clear to me exactly what the FEF25-75 is measuring, I am not sure there is such a thing as a “best” FEF25-75. Out of curiosity I reviewed a number of the older studies concerning FEF25-75 and although all the studies stated that their subjects performed multiple spirometry efforts I was interested to note that the FEF25-75 selection process was rarely, if ever, detailed. Of the exceptions, one stated the FEF25-75 was taken from the spirometry effort with the best FEV1 and another stated that the FEF25-75 was averaged from three efforts. The ATS/ERS statement on spirometry says that the FEF25-75:
“… is taken from the blow with the largest sum of FEV1 and FVC.”
This makes a certain amount of sense but because this statement is not referenced to any studies it should only be taken as a way to standardize the measurement of the FEF25-75 and not as a resolution about what constitutes the “best” FEF25-75. Even so it still leaves the door open to some varied interpretations. There are at least two situations where this is problematic. First, when two spirometry efforts have the same combined FVC + FEV1 value, and second, when an individual’s spirometry efforts are highly variable and the FVC and FEV1 have to be selected from separate efforts.
I didn’t have to go very far to find examples for both of these problems.