Peak expiratory flow and personal best in asthma

Peak expiratory flow (PEF) measures how fast 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 take the test only one time during the day, take it first thing in the morning, before using a bronchodilator medication. Record your PEF in your asthma diary(What is a PDF document?).

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%.

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

Personal best

Your personal best is your highest peak flow. This is the number you use to find your asthma zones. You use your personal best in your written plan that tells you what to do during a sudden increase in your asthma symptoms (asthma attacks). If you do not know your personal best, talk to your health professional.

You determine your personal best by taking PEF readings over 2 to 3 weeks when your asthma is under control—when you feel good and have no symptoms. During these weeks, you should record peak expiratory flows 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, medications may need to be changed or increased to prevent asthma from becoming worse. If personal best measurement improves significantly, the amount of medication may be reduced. Children need to have their personal best measurement checked about every 6 months to account for growth.