Peak expiratory flow monitoring

Peak expiratory flow (PEF) is the maximum rate at which air is exhaled during a forced expiration, expressed as litres per minute (L/min).

PEF is typically measured using a handheld peak flow meter, but can also be measured using a spirometer.

PEF measurements are effort-dependent, and are affected by the size of preceding inspiration, diameter of large airways, expiratory muscle strength and patient cooperation when performing the test. PEF measurements are unreliable in children.

Repeated peak flow measurements are useful for monitoring variable airflow obstruction in asthma, and can be used to assess response to therapy. Measurements should always be performed using the same peak flow meter because there is substantial variability in the results from different meters.

Peak flow meters should not be used in place of spirometry for diagnosing asthma because they cannot measure reversible airflow limitation or expiratory airflow limitation. However, if airflow reversibility is not shown on spirometry but the suspicion of asthma remains high, repeated peak flow measurements can be used to identify if there is variable airflow obstruction.

Peak flow meters are not useful in chronic obstructive pulmonary disease (COPD) because airflow obstruction is not variable.

For more information on peak expiratory flow monitoring in asthma and a downloadable peak flow chart for plotting PEF measurements against time, see the National Asthma Council website.