Asthma | Diagnosis & Tests


What is a peak flow meter?

A peak flow meter is a hand-held device that measures your peak expiratory flow rate (PEFR), or how fast you can blow air out of your lungs. Measuring your peak flow regularly can help you tell whether your asthma is getting worse.

To use a peak flow meter, you will first need to find out your "personal best" peak flow. Take a deep breath and blow as hard as you can into the mouthpiece. Your personal best is the highest reading you get on the meter over a 2-week period when your asthma is under good control.

How do I use a peak flow meter?

To use a peak flow meter, follow these steps:

  1. Move the indicator to the bottom of the numbered scale.
  2. Stand up.
  3. Take a deep breath.
  4. Close your lips (or have your child close his or her lips) around the mouthpiece of the flow meter. Your tongue should not go inside the tube.
  5. Blow out as hard and fast as possible.

The indicator on the meter will move up. Write down the number where it stops. Repeat steps 1 through 5 two more times. Write down the highest of the three numbers on the peak flow meter record chart.

Sample peak flow chart

Below is a sample of a peak flow meter record chart. You may mark your child's daily scores on a similar graph to see whether your asthma is in the green zone, yellow zone or red zone.

Adapted from "Teach your patients about asthma. A clinician's guide." Bethesda, Md.: National Heart, Lung, and Blood Institute, 1992; DHHS publication no. 92-2737.

What is the peak flow zone system?

Once you know your (or your child's) personal best peak flow score, your doctor can tell you how to do the next step. Peak flow scores are put in "zones" like the colors in traffic lights.

  • Green Zone: This is a score that is 80% to 100% of the personal best score. It signals that your/your child’s asthma is under control. No symptoms are present, but you/your child should take preventive asthma medicines as usual.
  • Yellow Zone: This is a score that is 50% to 80% of the personal best score. It signals that your/your child's asthma is getting worse. You may be coughing or wheezing frequently. You may need extra asthma medicine. Follow your doctor's written instructions or call your doctor for advice.
  • Red Zone: This is a score that is below 50% of the personal best score. It signals a medical emergency. You/your child may have severe coughing, wheezing and shortness of breath, and your lips and fingernails may be turning a grayish or bluish color.  Use an inhaler or other medicine to open your airways right away. Call your doctor immediately for more advice.

Some information adapted from "What you and your family can do about asthma," a patient information booklet published by the Global Initiative for Asthma, a joint effort of the National Heart, Lung, and Blood Institute and the World Health Organization. This and other publications are available through the Internet (

Written by editorial staff

Reviewed/Updated: 04/14
Created: 03/05