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 flowing out of the lungs, called peak expiratory flow rate (PEFR), as you forcefully blow into the device. A peak flow meter, when used properly, can reveal narrowing of the airways well in advance of an asthma attack. Used mainly by people with moderate to severe and persistent asthma, peak flow meters 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 doctor

  • What triggers the asthma attack (such as exercise-induced asthma)

What are peak flow zones?

Peak flow zones are based on the traffic light concept: red means danger, yellow means caution, and green means safe. Based on your personal best peak flow measurement (the optimal level of your lung function), your three peak flow 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 doctor.

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

The goal of the peak flow zones is to help you recognize when the asthma may start to become uncontrolled. The goal is to stay within the 80% value of your personal best peak flow measurement. Zones with a smaller range, such as 90% to 100%, may be recommended by some health care providers. Always follow the advice of your doctor regarding the peak flow rate.

The National Heart, Lung, and Blood Institute (NHLBI) recommends that the numbers for each of your peak flow levels be marked on your meter with colored tape. The numbers should also be added to your written Asthma Action Plan along with your doctor's advice on both the medicines and the steps you should take in each peak flow zone. The steps could be such things as what medications 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 number. 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.

After recording your peak flow measurements for two to three weeks consistently, your doctor may also measure your optimum lung function with a spirometer (a device that can check lung function by measuring both the amount of air expelled and how quickly the air was expelled). The spirometer measurement may then be compared with the peak flow meter record to help set up an asthma management and treatment plan.

Your personal best peak flow measurement may change over time. Consult your doctor as to when to check for a new personal best peak flow measurement.

When should a peak flow meter be used?

Peak flow meters should be used regularly to check how well your asthma is being controlled. In addition, the peak flow meter may be a valuable tool during an asthma attack, because it can help determine how well your short-term, quick-relief asthma medication is working. NHLBI recommends measuring lung function at the following times:

  • Every morning, before taking asthma medications

  • During asthma symptoms or an asthma attack

  • After taking medication for an asthma attack

  • Other times recommended by your doctor

However, the peak flow meter is a tool to help collect information. The key to successful asthma management is communicating this information (the peak flow meter recordings, the severity of your symptoms, and the effectiveness of your medications) to your doctor.

How can I get a peak flow meter?

Peak flow meters are available over-the-counter and without a prescription. However, always consult your doctor about getting and using a peak flow meter and for step-by-step instructions on how to use the device.

If you are using more than one peak flow meter, be sure they are the same brand.

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