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Asthma: Peak Expiratory Flow and Personal Best

British Columbia Specific Information

Asthma is a common chronic condition that causes your airway to narrow and swell. You may experience symptoms such as shortness of breath, tightness in the chest, coughing, and wheezing. Symptoms may range from being mildly annoying to deadly. For this reason it is important to speak with a health care provider if you think you may have asthma.

For more information about asthma, visit our Asthma Health Feature. You may also call 8-1-1 to speak with a registered nurse 24 hours a day, 7 days a week.​


What are peak flow and personal best?

Peak expiratory flow, or peak flow, is how much air you breathe out when you try your hardest. You measure peak flow with a peak flow meter, a device that you can use at home.

  • If you can breathe out quickly and with ease, you have a higher number. This means you have a higher peak flow. Your lungs are working well, and your asthma may not be bothering you.
  • If you can only breathe out slowly and with difficulty, you have a lower number. This means you have a lower peak flow. Your lungs aren't working well, even if you aren't having asthma symptoms.

A person's personal best is their highest peak flow reading. You can find your or your child's personal best by taking peak flow 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 flows should be recorded at least 2 times a day. Your personal best is the highest reading you have over this period of time.

Why are they measured?

Peak flow is used in the monitoring and treatment of asthma to find out how well your or your child's lungs are working. 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 start. Peak flow rates are compared with charts that list normal values for sex, race, and height.

The highest peak flow reading—a person's personal best—is the number the doctor uses to set up the zones in an asthma action plan. For example, the green zone is 80% or more of the personal best reading. The yellow zone is 50% to 79% of the personal best reading. The written action plan lists what steps to take for each zone.

The personal best measurement needs to be evaluated now and then to see if it has changed. If it decreases, medicines may need to be changed or increased to prevent asthma from becoming worse. If the personal best measurement improves a lot, the amount of medicine may be reduced. Children need to have their personal best measurement checked about every 6 months to account for growth.


Current as of: November 14, 2022

Author: Healthwise Staff
Medical Review:
John Pope MD - Pediatrics
Adam Husney MD - Family Medicine
Elizabeth T. Russo MD - Internal Medicine