What is a peak flow meter?

A peak flow meter is a device that measures how fast air comes out your lungs when you blow out as hard and as fast as you can. This is called peak flow, or "PF" and is measured in liters per minute (lpm).

Peak flows measure how open the large airways are in the lungs. Peak flow values may drop hours or even days before asthma symptoms occur, alerting you that asthma may be getting worse even though you seem to be fine. By taking medicine early (before symptoms), you might be able to stop the episode quickly and avoid a severe asthma episode. The peak flow meter can also be used to help you:

  • Learn what triggers asthma;
  • Decide when to add or stop medicines;
  • Know when to seek emergency care.

How do I use the peak flow meter?

A peak flow meter is simple to use. Even children ages 4 and up should be able to perform a PF with good results.

To perform a peak flow:

  1. Stand up straight.
  2. Make sure the indicator is at the bottom of the meter.
  3. Take a deep breath, filling your lungs completely.
  4. Place the mouthpiece in your mouth; lightly bite with your teeth, and close your lips on it.
  5. Blast the air out as hard and as fast as possible in a single blow.
  6. Record the number that appears on the meter.
  7. Repeat these steps 3 times.
  8. Record the highest of the 3 readings in an asthma diary. This reading is your or your child's peak flow.

Finding your "personal best" peak flow

The "personal or usual best" peak flow is the highest number you or your child can perform over a 2-week period when asthma is under good control. Good control means you feel good and do not have any symptoms. The personal best PF is the number to which all other peak flow readings will be compared.

There are predicted PF values based on age, height, and gender. These numbers may be helpful, but your or your child’s personal best is more important.

To find your personal best peak flow, perform peak flows:

  • Twice a day for 2 weeks;
  • At the same time in the morning and in the early evening;
  • Before taking any inhalers, or as instructed by your caregiver.

Once you have determined your or your child's personal best PF, continue to perform peak flows daily or as instructed by your caregiver. Regular use of peak flow will help you recognize early drops in airflow and guide your asthma care, using a written treatment plan called the Asthma Action Plan. Especially in children, regular use tells when your child’s personal best is improving naturally as he or she grows.

If your or your child’s PF drops below 80% of your personal best, follow your Asthma Action plan and check your PF more frequently that day (or as directed by your caregiver).

Using your Asthma Action Plan

Keeping a daily record of your PFs and understanding how this relates to changes in asthma will help you to better manage asthma episodes. To help patients understand how this works, caregivers use the “traffic light” system.

In the Asthma Action Plan, there are three zones: Green, Yellow, and Red.

  • Green zone: Asthma is well-controlled. PF is 80% to 100% of your personal best.
  • Yellow zone: Asthma is flaring up or is poorly controlled. PF is 50% to 80% of your personal best.
  • Red zone: Asthma is severe; requires emergency care. PF is less than 50% of your personal best.

To find the number ranges for your asthma zones, multiply your personal best PF by 1 (100%), .8 (80%), and .5 (50%). Record these ranges in an asthma diary so that you can refer to them easily.

For example, if the personal best PF is 400 lpm, the zones will be:

  • Green zone: 320 to 400 lpm (320 [400 x.8] to 400 [400 x1])
  • Yellow zone: 200 to 320 lpm (200 [400 x .5] to 320 [400 x .8])
  • Red zone: Less than 200 lpm (400 x .5)

If you are in the Yellow or Red zone, you will need to adjust your medicine according to your caregiver’s instructions. You will use the PF values and symptoms to take actions. Make sure you understand and can follow the plan. Ask your caregiver any questions you might have.