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For Parents: Your Child’s Peak Flow Monitoring

A peak flow meter is a tool for testing your child’s lungs. It can help warn you of a flare-up, even before there are symptoms. Your child’s health care provider will tell you whether to use the peak flow meter, and if so, how often. He or she should also check that your child is using the peak flow meter correctly.

The Peak Flow Meter

A peak flow meter measures how much air your child can quickly push out of the lungs. This helps show how open the child’s airways are at that moment. Your child’s peak flow meter may look different from the one shown here, but will work in a similar way.


Move the marker to zero or the lowest number on the scale. Have your child stand if possible. Ask your child to take as deep a breath as she can.

Put the mouthpiece of the meter in your child’s mouth. Ask your child to blow into the mouthpiece once, as hard and fast as possible.

Check where the marker has moved on the numbered scale. Write down this number. Move the marker back to zero and repeat the test two more times. The highest of the three is your child’s peak flow number.

What Is the Personal Best?

Your child’s personal best is his or her highest peak flow number during a week or two with no symptoms. Other peak flow results are compared to the personal best. This helps show how your child is doing over time.

What Do Peak Flow Numbers Mean?

A peak flow number lower than 80 percent of the personal best may signal a flare-up. Keep in mind that peak flow can vary from day to day. Other factors may also affect peak flow:

  • Age. Lungs grow as a child grows. So the personal best peak flow number should increase as the child gets older.

  • Control. The personal best may increase once asthma is in control.

  • Poor effort. Make sure your child gives a good effort.


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