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Asthma: Measuring Peak Flow

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As someone with asthma, you know how important it is to monitor your condition. You need to know how well your lungs are "working." Is their ability to move air in and out staying the same, or is it getting better or worse?

When you monitor your asthma, you can control it. When you control your asthma, you also control your life—you do what you want to do, and your asthma does not limit you.

Measuring your peak expiratory flow is an important part of monitoring your asthma.

Peak expiratory flow (PEF) measures how much air you breathe out when you exhale. It tells you how well your lungs are working. You measure PEF with a peak flow meterClick here to see an illustration., an inexpensive device that you can use at home.

  • If you can breathe out quickly and with ease, you will have a higher number (higher peak flow rate). Your lungs are working well, and your asthma is not bothering you.
  • If you can only breathe out slowly and with difficulty, you will have a lower number (lower peak flow). This usually means that your lungs are not working well, even if you don't have any usual asthma symptoms.

You measure PEF as liters of air per minute.

How often should I measure PEF?

How often you measure your PEF depends on how severe your asthma is and how often you have asthma attacks. If you have severe asthma or cannot tell when you are having asthma symptoms, you may need to check your PEF twice a day, in the morning and in the evening. If you have mild asthma, you may not need to check your PEF daily and can instead monitor it by paying attention to symptoms. But if any asthma symptoms develop, you may need to check your PEF. Talk with your doctor about how often you check your PEF.

Test Your Knowledge

What does PEF measure?

PEF measures how much air you breathe out when you exhale.
PEF measures how many breaths you take in 1 minute.
PEF measures how much air your lungs can hold.

Measuring your PEF is important, because it lets you:

  • Determine your asthma zones, which you use in your asthma action plan. During an acute asthma attack, the zone you are in determines your medicine and action.
  • Know whether an acute asthma attack is going to occur and how severe it may be. If you know you are going to have an asthma attack, you can take medicine to prevent it or make it less severe. This may help you avoid having to go to the emergency room.
  • Identify things that may trigger an asthma attack, such as pollen, cigarette smoke, or dust mites.
  • Measure changes in your breathing. This can help your doctor to:
    • Decide whether you need to change, increase, or decrease the long-term medicine used in your daily asthma treatment plan.
    • Tell which medicines are helping your breathing and which are not.

Test Your Knowledge

Measuring your PEF can help you know if you may have an asthma attack.


If you have never used a peak flow meterClick here to see an illustration., talk with your doctor about how to use it correctly, and then practice using it.

Measure your PEF regularly, even if you are feeling good. PEF is lowest in the early morning and highest in the afternoon. When you measure your PEF once a day, it needs to be done first thing in the morning before you use your asthma medicine.

It's very important to record the results of your PEF measurements in your asthma diary. This will help you notice changes in your breathing. Take your asthma diary with you when you see your doctor so you can review it together. It's very important to review the diary with your doctor whenever you feel your lung function is getting worse.

Measuring your peak expiratory flow

Remove any gum or food you may have in your mouth. Then use your peak flow meter to:

Measure your peak expiratory flowClick here to see an illustration..

After you have blown into the meter 3 times, take the highest number you received, and write it in your asthma diary or on another record sheet.

If you cough or make a mistake during the testing, do the test over.

Different brands of meters may give different values for results. If you change meters, you will need to determine your asthma zones using the new meter.

Note: If your best effort is in your red zone, take your relief medicine immediately and call your doctor or go to the emergency room.

Test Your Knowledge

When you measure your PEF once a day, it needs to be done first thing in the morning before you use your asthma medicine.


Talk with your doctor

If you have questions about this information, take it with you when you visit your doctor. Make notes about what you would like to ask about or discuss.

If you would like more information on asthma, the following resource is available:


Asthma and Allergy Foundation of America (AAFA)
1233 20th Street NW
Suite 402
Washington, DC 20036
Phone: 1-800-7-ASTHMA (1-800-727-8462)
Email: [email protected]
Web Address:

The Asthma and Allergy Foundation of America (AAFA) provides information and support for people who have allergies or asthma. The AAFA has local chapters and support groups. And its Web site has online resources, such as fact sheets, brochures, and newsletters, both free and for purchase.

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ByHealthwise Staff
Primary Medical ReviewerJohn Pope, MD - Pediatrics
Specialist Medical ReviewerLora J. Stewart, MD - Allergy and Immunology
Last RevisedMarch 14, 2013

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