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Understanding FEF25-75



I had a required Company Physical for my job that included a lung function test. The best result I got on the FEF25-75 was 43%. I asked what that meant and they looked in my chart and said not to worry it was probably just because of my Asthma. I do have Asthma but I was not having problems when I took the Physical. I asked if I could use my inhaler and try again, they said no because it was not a diagnostic or treatment setting, it was just a test. I always thought that people with asthma had normal lung function when they were not having asthma symptoms. What is FEF25-75 and why would it be so low when I was not sick?


Asthma is a disease characterized by narrowing of the airways. This will lead to airflow obstruction that can be measured on Pulmonary function testing. The most reproducible is the FEV1 (forced expiratory flow in one second) that actually measures flow in large or promixal airways. The most convient for self measurement is the PEF (peak expiratory flow) and this has been incorporated into most patient`s asthma action plan. The FEF 25-75 measures flow from the first quarter of the expired lung volume to the third quarter of the expired lung volume. While it may be a better reflection of the more distant airways, it has a much broader range of normal than the other measures. If you`re not having asthmatic symptoms it would probably not be a measure requiring further treatment. If you have further concerns you should relate them to the physician treating your asthma.

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Response by:

Mitchell C Rashkin, MD Mitchell C Rashkin, MD
Professor of Medicine
College of Medicine
University of Cincinnati