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Peak Flow Meter

Reviewed by: HU Medical Review Board | Last reviewed: November 2021

A peak flow meter is a handheld device you use to monitor your asthma at home. It measures how well air moves out of your lungs in 1 large breath. This is called your peak expiratory flow.1

Your peak flow numbers tell you how well your asthma is controlled. The meter also can warn you about a possible asthma attack. It is used every day by adults and children over age 5.1

Your doctor will teach you how to use your peak flow meter. They can explain when and how often to use it, and train you to understand what the numbers mean. Your Asthma Action Plan will also tell you what the measurements mean and what actions take based on those numbers.

How to use a peak flow meter

To use your peak flow meter, stand up straight and take a deep breath. Place the meter in your mouth and breathe out as hard as possible in 1 breath. Repeat this process 2 more times and write down the highest number.1,2

You should use your peak flow meter around the same time every day. Many people use it twice daily. But you may use it differently, depending on what your doctor says is right for you. You may also want to use it before or after using asthma medicine.1

How a peak flow meter helps with asthma

Large changes in your peak flow are a sign of poor asthma control since this test is very sensitive to changes in your airways. Your peak flow decreases when your airway narrows. It can alert you hours, or even days, before you feel asthma symptoms.1

If you can detect airway narrowing early, you can start taking medicines right away. This may help prevent severe asthma episodes.1

A peak flow meter can sometimes detect changes in your asthma before you or your doctor. Some ways a peak flow meter might help you or your doctor include:3

  • Seeing how you respond to a new treatment
  • Identifying triggers that cause an asthma attack
  • Monitoring changes that indicate an asthma attack is starting
  • Monitoring recovery after an asthma attack

A peak flow meter is not used to diagnose asthma, nor is it used to measure asthma severity. Other tools are used for this, such as breathing tests like spirometry.3

Things to know

Usually, you will measure your peak flow at least twice a day for 2 to 3 weeks when your asthma is well-controlled. Your personal best will be the highest measurement over this period. Everyone will have a different “personal best” based on age, height, sex, and other factors. Your doctor or nurse can teach you how to find your personal best.1,2

Your Asthma Action Plan will tell you what to do depending on your peak flow numbers. Your plan may show a chart with zones that correspond to a traffic light. For example:1,2

  • Green zone (80 to 100 percent of your personal best): your asthma is under control, and you should take your asthma medicines as usual.
  • Yellow zone (50 to 80 percent of your personal best): you may be having an asthma episode that requires quick-relief drugs or other medicines.
  • Red zone (less than 50 percent of your personal best): take your quick-relief medicine and call your doctor or go to an emergency department.

Other important things to remember about peak flow meters include:1

  • Record your peak flow numbers in an asthma diary every day
  • A drop in your peak flow of more than 20 percent may mean the start of an episode
  • Check your Asthma Action Plan for when to use your peak flow meter and how to interpret results
  • Use the same meter each time

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