FEF25-75
FEF25-75 is the average speed of airflow during the middle half of a forced breath out — specifically between the points when 25 percent and 75 percent of the vital capacity has been exhaled. While FEV1 captures the powerful early rush, this number listens to the quieter middle of the blow, where the smaller airways do their breathing.
Because the tiny peripheral airways contribute most of their resistance during this mid-expiratory phase, FEF25-75 is often touted as an early or sensitive marker of small-airway disease, sometimes falling before FEV1 or the ratio change.
Its sensitivity comes at the cost of reliability: FEF25-75 is highly variable, strongly dependent on the FVC and on effort, and has very wide normal ranges. For these reasons modern guidelines treat it with caution and do not recommend it as a standalone basis for diagnosis — it is a hint to weigh, not a verdict.
A young smoker with a normal FEV1/FVC ratio shows an FEF25-75 of 60 percent predicted, raising suspicion of early small-airway disease to monitor over time.
FEF25-75 may dip early in small-airway disease but is too variable to stand alone.
Because FEF25-75 is measured at fixed fractions of the FVC, a changing FVC shifts the volume range being sampled, so before-and-after comparisons can be misleading.