Health Library

Peak Flow Meter

What is a peak flow meter?

A peak flow meter is a device used to measure how well your asthma is under control.

The device measures air flow out of the lungs (peak expiratory flow rate or PEFR), as you blow into it. A peak flow meter, when used properly, can show narrowing of the airways before you have symptoms. Peak flow readings can help determine:

  • When to seek emergency medical care

  • How well your asthma management and treatment plan are working

  • When to stop or add medication, as directed by your health care provider

  • What triggers asthma symptoms

What are peak flow zones?

Peak flow zones may be part of your asthma action plan. If you don't have an asthma action plan, or if yours isn't up-to-date, make sure you talk with your health care provider. The 3 zones tell you when your asthma is controlled, when it is getting worse, and when it is severe. They are based on the traffic light concept. Green means safe. Yellow means caution. Red means danger. Based on your personal best peak flow measurement (your best lung function), your 3 peak zones include:

  • Green: 80% to 100% of your personal best peak flow measurement. This means your asthma is under control.

  • Yellow: 50% to 79 % of your personal best peak flow measurement. This is a sign that your asthma is getting worse. You may need to use quick-relief medications or other medication, as directed by your health care provider.

  • Red: Below 50% of your personal best peak flow measurement. This is a medical emergency.  You should take quick-relief medication and seek medical help right away.

Using peak flow zones helps you recognize when your asthma may start to become uncontrolled. The goal is to stay in the green zone. Zones with a smaller range, such as 90% to 100%, may be recommended by some health care providers.

The National Heart, Lung, and Blood Institute (NHLBI) recommends that the numbers for each of your peak flow levels be marked on your meter. If you need help with this, ask your provider or nurse. Your provider will also include your usual symptoms and what you should do for each zone as part of your asthma action plan. The steps could be such things as what medication to take, when to call the doctor, or when to go to the emergency room. 

How is a personal best peak flow measured?

Your peak flow zones are based on your personal best peak flow. To figure out  your personal best peak flow, take your peak flow measurement each day at the same time (middle of the day) for two to three weeks, when your asthma is under control. Write down all of the readings. Your personal best is the highest number during this time.

Your personal best peak flow measurement may change over time. Talk with your provider about when you should recheck your personal best.

When should a peak flow meter be used?

Peak flow meters should be used regularly to check how well your asthma is being controlled. NHLBI recommends measuring peak flow at the following times:

  • Every morning, before taking asthma medications

  • When you have symptoms or an asthma flare-up

  • After taking medication for an asthma flare-up

  • Other times as recommended by your provider

Make sure you share your peak flow readings with your health care provider.

How can I get a peak flow meter?

Peak flow meters are available at most drug stores or pharmacies. Some providers may also have them in their offices. Make sure you read all instructions that come with the meter. If you have any questions, ask your provider or nurse. The best way to know if you are using the meter correctly is to use it in front of your provider or nurse.

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