Topic Overview

Peak expiratory flow (PEF) measures how much air you or your child can breathe out using the greatest effort. It is used in the monitoring and treatment of asthma to determine how well your lungs are functioning. Your peak flow drops when the tubes that carry air to the lungs (bronchial tubes) narrow. A decrease in the peak flow can show that the bronchial tubes have narrowed even before asthma symptoms develop. Peak expiratory flow is measured with a peak flow meter, an inexpensive device that can be used at home.

Peak expiratory flow is lowest in the early morning and highest in the afternoon. If you or your child takes the test only one time during the day, take it first thing in the morning, before using a bronchodilator medicine. Record the PEF in your asthma diary .

PEF varies throughout the day. In a person who does not have asthma, it may vary between 10% and 15%. In a person who has poorly controlled asthma, it may vary more than 20%.

Peak expiratory flow rates are compared with charts that list normal values for sex, race, and height.

Personal best

A person's personal best is his or her highest peak flow. This is a number you can use to find the correct asthma zones. The personal best may be used in the written plan that says what to do during a sudden increase in asthma symptoms (asthma attacks).

You determine your or your child's personal best by taking PEF readings over 2 to 3 weeks when the asthma is under control—when you or your child feels good and has no symptoms. During these weeks, peak expiratory flows should be recorded at least twice daily. The personal best is usually reached in the afternoon or evening. Personal best measurements are never measured during an asthma attack.

Your personal best is the highest reading you have over this period of time.

Personal best measurement needs to be evaluated once in a while to see whether it has changed. If it decreases, medicines may need to be changed or increased to prevent asthma from becoming worse. If personal best measurement improves significantly, the amount of medicine may be reduced. Children may need to have their personal best measurement checked about every 6 months to account for growth.