What is peak flow?

Peak flow is how much air you breathe out when you try your hardest. You measure peak flow with a peak flow meter, an inexpensive device that you can use at home, if your doctor has recommended this device as part of your Asthma Action Plan.

• If you can breathe out quickly and with ease, you have a higher number. This means you have a higher peak flow. Your lungs are working well, and your asthma may not be bothering you.
• If you can only breathe out slowly and with difficulty, you have a lower number. This means you have a lower peak flow. Your lungs aren't working well, even if you aren't having asthma symptoms.

Why measure your peak flow?

It's important to know how well your lungs are working. One way to do this is by checking your peak flow with a peak flow meter. Your peak flow can tell you if your asthma is staying the same, getting better, or getting worse. It helps you know if you need to take action.

How to test peak flow

Before you start, remove any gum or food you may have in your mouth.

1. Set the pointer.

   Be sure the gauge of the peak flow meter is set to 0 or the lowest number on the meter.

2. Attach the mouthpiece to the meter.

   Some meters don't have a separate mouthpiece.

3. Sit up or stand up as straight as you can, and take a deep breath.
4. Close your lips tightly around the mouthpiece.

   Keep your tongue away from the mouthpiece.

5. Breathe out as hard and as fast as you can for 1 or 2 seconds.

   A hard and fast breath usually makes a "huff" sound.

6. Write down the number on the gauge.

   This is your peak flow.

7. Repeat these steps 2 more times.

   Write down the highest of the three numbers in your asthma diary.

If you cough or make a mistake during the testing, redo the test. If your best effort results in a number you didn't expect, follow the instructions included with your device, redo the test, and make sure you are following the proper technique.

How do you use peak flow to manage asthma?

Your action plan is based on zones of asthma severity. Your peak flow can help you find out what zone you are in. You do this by comparing your current peak flow to your personal best peak flow.

Your personal best is your highest peak flow recorded over a 2- to 3-week period when your asthma is under control.

• Green zone. Green means good. You want to be in the green zone every day. You are in the green zone if your peak flow is 80% to 100% of your personal best. To figure 80% of your personal best, multiply your personal best by 0.80. For example, if your personal best flow is 400, multiplying by 0.80 gives you 320. So if your personal best is 400, you are in the green zone as long as your peak flow is 320 or higher.
• Yellow zone. Yellow means caution. You are in the yellow zone if your peak flow is 50% to 79% of your personal best. To figure 50% of your personal best, multiply your best flow by 0.50. For example, if your personal best flow is 400, multiplying by 0.50 gives you 200. Your asthma action plan will tell you what to do when you are in your yellow zone.
• Red zone. Red means STOP. You are in the red zone if your peak flow is less than 50% of your personal best. Your symptoms may be severe, and you may have extreme shortness of breath and coughing. Get medical help right away, and follow your action plan. You may need emergency treatment or admission to a hospital.

Each meter is a little different. If you change meters, you will need to find your asthma zones using the new meter.