Your Child’s Asthma: Peak Flow Monitoring

Asthma symptoms can be monitored by closely watching for early changes or by using a peak flow meter. A peak flow meter is a tool for testing how well your child’s lungs are working. It can help warn you of a flare-up, even before there are symptoms. Make sure you know when you and your child should check his or her peak flow. And make sure that you and your child know how to use the 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.

Three steps of child using peak flow meter.

Steps for checking peak flow:

  • 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 possible.

  • You should put the mouthpiece of the meter in his or her 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.

What is the "personal best"?

Your child’s personal best is his or her highest peak flow number during a week or two when he or she is not having 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% 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 should increase as the child gets older.

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

  • Poor effort. Make sure your child takes a deep breath and exhales as quickly and as hard as possible.