Forced expiratory flow (FEF)

Forced expiratory flow (FEF) is the speed that someone can breathe out air from their lungs, measured in the middle of a hard, fast breath out.

Forced expiratory flow (FEF) is a way of checking how well the lungs work. To measure FEF, a person takes the deepest breath in they can manage, then blows out the air as hard and fast as they can. A machine measures the air flow, or how fast the person exhales.

FEF is the air flow from the time where a quarter of the air is breathed out to the time when three quarters is breathed out. Sometimes FEF is written as FEF25-75%.

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