Peak expiratory flow (PEF) is a measure of how fast a person can exhale while breathing out as hard and fast as possible. PEF is used to evaluate the condition of the airways in people who have asthma or other conditions that affect the ability to breathe.
When the airways become narrowed because of inflammation or disease, the peak expiratory flow drops. A decrease in the peak rate can show that the bronchial tubes have narrowed even before symptoms of asthma occur.
Peak expiratory flow may be measured at home using an inexpensive device called a peak flow meter, or it may be measured in a doctor's office using a spirometer.
WebMD Medical Reference from Healthwise