Peak Flow Meter

Reviewed by: HU Medical Review Board | Last Reviewed: May 2016. | Last updated: November 2020

If you have asthma, you can use a peak flow meter to measure your lung function, or peak expiratory flow (PEF), at home. The peak flow meter is a handheld, portable device that measures how well air moves out of your lungs.1 Your peak flow meter can be used for short-term monitoring in certain situations. It also can be useful for long-term daily monitoring.

Why should I measure my asthma PEF?

Short-term monitoring is useful for many reasons, such as:2

  • Starting or changing medications, to see how well they are working.
  • Monitoring an increase in asthma symptoms that indicate an attack is starting.
  • Monitoring your recovery after an asthma attack
  • Identifying triggers at work or in other environments.
  • Monitoring pregnancy-related changes in lung function at prenatal visits.

Long-term daily monitoring can indicate airway narrowing before an asthma attack starts. Airway narrowing starts hours—and sometimes days—before symptoms begin.1 If you can detect airway narrowing early, you can start taking your medications right away. You may be able to avoid a severe asthma attack. Daily monitoring may be useful for people who:1,2

  • Have difficult-to control moderate or severe asthma.
  • Have had a sudden, severe asthma attack.
  • Have a hard time noticing that their symptoms are getting worse.
  • Want to monitor their lung function.

The peak flow meter is not used to diagnose asthma or to measure how severe asthma is.1 Using a peak flow meter is one of the many ways that patients and their families manage their asthma. Adults and children ages five and older can use a peak flow meter.

How do I use my peak flow meter?

Your health care provider can provide you with instructions on how to use your peak flow meter. It is a good idea to bring your peak flow meter to your appointments and review how to use it correctly.

Instructions for using your peak flow meter

  • Move the indicator to the bottom of the numbered scale.
  • Stand up.
  • Take a deep breath, filling your lungs completely.
  • Place the mouthpiece in your mouth and close your lips around it. Do not put your tongue in the hole.
  • Blow out as hard and fast as you can in a single blow. If you cough or make a mistake, do not count this attempt.
  • Write down the number you get.
  • Repeat steps 3 through 6 two more times. Write the best measurement in your asthma diary. (Do not calculate the average the three attempts.) If your three measurements are close, it is a sign that you are using your meter correctly.1

Use the same meter each time. Measurements taken on different meters can vary by as much as 20%.2 Step-by-step instructions for using your meter are in Table 1.1 Measure your peak flow in the morning before taking your medications.

Graph of Normal Values for Peak Expiratory Flow for men and women, peaking around age 35 to 40

How do I use my asthma PEF results?

Your peak flow meter measures your peak expiratory flow (abbreviated PEF). One sign that your asthma is well controlled is that the PEF is similar each time. Some variation in lung function is normal for people with healthy lungs and people with asthma. However, a change of more that 15% indicates poor asthma control.3

You can compare your current PEF with your personal best PEF to see where your asthma is at. Your personal best PEF is the highest reading in a two-week period. Instructions for calculating your personal best PEF are in Table 2.1

Instructions for finding your personal best PEF

  1. Measure your PEF twice a day for 2 to 3 weeks. Take one reading in the early morning and another in the late afternoon or evening.
  2. Measure your PEF 15 to 20 minutes after taking your rescue inhaler.
  3. Measure your PEF at other times, if your doctor tells you to.
  4. The highest measurement in the 2 to 3 week period is your personal best PEF.1

Once you know your personal best PEF, you and your provider can use this information to develop a written asthma action plan. Asthma action plans describe what to do in the event of an asthma flare-up. Your plan should say who and when to call based on your peak expiratory flow measurements and the type of symptoms you are having.

Asthma action plans have three zones that correspond to a traffic light: Green, Yellow, Red.1

  • Green zone: Your PEF is 80%-100% of your personal best, meaning your asthma is under control. Take your medicines as usual.
  • Yellow zone: Your PEF is 50% to 80% of your personal best. Caution--Your asthma is worsening. Measure your peak flow a few times. If you stay in the yellow zone, use your rescue inhaler. Call your doctor to talk about making change to your medications.
  • Red zone: Your PEF is less than 50% of personal best. Danger. Call your doctor or emergency department, or go right to the emergency department.

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