Peak expiratory flow is the maximum speed at which a person can exhale air from the lungs in a single breath. Lung function tests allow this rate to be measured as an indicator of how airways are functioning. This data can help patients and physicians determine how diseases such as asthma and chronic obstructive pulmonary disease (COPD) are affecting the lungs at any given time. Peak expiratory flow readings are taken when the patient is exhaling as hard and as fast as possible.

Peak flow meters are small, hand–held devices that allow patients to monitor changes in the status of their airways. Patients breathe into the mouthpiece at one end and receive a reading on a scale with an indicator. Lower readings correspond with higher airway obstruction.

Patients should regularly use a peak flow meter to monitor their airways, regardless of how they are feeling. A reading that is 20 percent below normal, or a change from the patient's usual range, can warn of an impending asthma attack or other looming respiratory distress. This can be a warning sign to patients to take certain medications or to contact their physician.

Patients should record the results of peak flow meter tests in an asthma diary that tracks their condition. The result of any test can be compared to a personal best score, which is the highest peak–flow measurement recorded over a two–week period when a condition is under control. Patients have their condition under “good control” when they feel good and are not showing symptoms.