The peak expiratory flow rate measures how fast a person can exhale air. It is one of many tests that measure the function of the airways, which are commonly affected by diseases such as asthma, or chronic obstructive pulmonary disease (COPD, usually a combination of emphysema and chronic bronchitis).

In these lung diseases, air flow during exhalation is decreased by narrowing or blockage of the airways. The severity of asthma or COPD can change with time, and peak expiratory flow monitoring is used by many patients to monitor their lung function at home. This allows them to anticipate when their breathing will become worse and to take appropriate medications or call their health care providers before symptoms become too severe.

Normal values vary considerably according to a person's age, sex, and size. Peak flow measurements are most useful when a person is able to compare the peak flow obtained on a day-to-day basis. A fall in peak flow, especially when accompanied by symptoms such as increased cough, shortness of breath, or wheezing, may signal the onset of a flare of lung disease, requiring early treatment to prevent complications.

Peak expiratory flow rate measurements are not as accurate as the spirometry measurements performed in a health care provider's office. Small changes in your peak flow may not mean significant changes in your lung function.