Peak expiratory flow is the maximum air speed a person can achieve during forced exhalation that begins with the lungs fully inflated. Many patients with asthma, chronic obstructive pulmonary disease (COPD) or other respiratory disorders regularly measure this ability to indicate how well their airways are functioning. Lower readings may indicate an impending asthma attack or other looming respiratory distress before symptoms develop. In fact, the device can warn of an attack that is two or three days away.    

Peak expiratory flow readings are taken when the patient is exhaling as hard and as fast as possible. Patients measure their peak expiratory flow rate (PEFR) with a peak flow meter. People with moderate to severe breathing problems should measure their PEFR regularly, and should carry their meter with them at all times.  

Peak flow meters are also used to measure peak flow variability. This is a measure of the change in peak flow readings from test to test. Patients who have symptoms under control should have peak flow readings that consistently vary less than 15 percent.

PEFR readings are used in conjunction with asthma action plans, which are written, color–coded guidelines on when to use medication and/or seek medical care. An asthma action plan is prepared with the help of a physician. It compares a patient's best reading with their current reading.

Patients may also use these readings to uncover the effects of allergens or irritants on their breathing ability. For example, peak flow readings may drop during seasons when airborne allergens such as pollen are prevalent.

Peak flow meters are available through a healthcare provider or at a local pharmacy. They are easy to use for most people, and can generally be used by children ages 5 and older. Some children may be able to use the meter at age 3. While the standard range peak flow meter is reliable for older children, teenagers and adults, young children have smaller airways and often require a low range meter. Parental supervision of peak flow measurements is important for evaluating the condition of a child’s asthma.

It is important to use the same peak flow meter for each test. Studies have shown that measurements recorded by different brands of peak flow meters vary widely from one another. Patients who switch to a different peak flow meter will have to establish a new personal–best baseline, which is the highest peak-flow measurement recorded over a two-week period when a condition is under control. This score will serve as a benchmark against which to measure asthma progress. The different expiratory rate is what is important to measure during this test; therefore it is necessary to know whether rate changes are due to a new peak flow meter or actual airflow changes.

Patients should record the results of peak flow meter tests in an asthma diary that tracks their condition. Patients should bring their asthma diary to any visits with their physician. Examining information in the diary can help a physician determine the effectiveness of a treatment plan and whether or not any changes are necessary. The recorded PEFR measurements can also be compared to readings taken with equipment in the physician's office to ensure accuracy.