Peak expiratory flow, or PEF, is the amount of air you can quickly expel from your lungs. Used to measure lung function in people with asthma, PEF can be tested at home by blowing out as hard and as quickly as you can into a device known as a peak flow meter.
Physicians may also measure the amount of air you can expel from your lungs as part of an asthma diagnosis or checkup. Doctors gauge this using spirometry, which involves exhaling as much air as you can, as fast as you can, into a tube attached to a device called a spirometer. This results in a more specific measure known as FEV subscript1, or forced expiratory volume in one second.
Because asthma is a respiratory condition in which the airways in the lungs are narrowed and inflamed, being able to expel more air—meaning that lung tissue is less swollen and inflamed—is better than expelling less air, which can indicate that lungs are inflamed due to asthma.