Asthma: Measuring 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 do you measure your peak expiratory flow?
To perform the peak expiratory flow (PEF) test, you need a peak flow meter. A peak flow meter is an inexpensive handheld device you breathe into as hard and as fast as you can.
Read and follow the instructions included with the peak flow meter. Ask your doctor to show you how to use this device before you use it at home. If you have questions about how to use a peak flow meter or how to read the results, talk with your doctor.
If you use medicine to help with breathing (such as for asthma), talk to your doctor to learn how long you should wait to test your lung function after taking your medicine. You may need to wait a few hours after taking the medicine to do the test. Or your doctor may recommend that you test your lung function in the morning before you take your medicine.
Avoid eating a heavy meal before performing a PEF test. Be sure to sit up or stand up as straight as possible to help you take as large a breath as you can. Use the same position every time you test your PEF. Peak flow monitoring relies on your trying as hard as you can. For accurate results, be sure to give the test your best effort every time.
Measuring your peak flow
Set the meter to its lowest number: Step 1
Put the pointer on the gauge of the peak flow meter to 0 or the lowest number on the meter. In the photo, this is about 50.
Attach the mouthpiece: Step 2
Attach the mouthpiece to the peak flow meter. Some meters don't have a separate mouthpiece.
Take a deep breath: Step 3
While standing, take a deep breath.
Put the mouthpiece in your mouth: Step 4
Put the peak flow meter mouthpiece in your mouth, and close your lips tightly around the outside of the mouthpiece. Don't put your tongue inside the mouthpiece.
Breath out hard: Step 5
Breathe out as hard and as fast as you can for 1 or 2 seconds. A hard and fast breath usually produces a "huff" sound.
Check your number: Step 6
Check the number on the gauge, and write it down. This is your peak expiratory flow (PEF). In the photo, the PEF is about 180.
Repeat these steps two 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.
Current as of: November 14, 2022
Author: Healthwise Staff
Medical Review:John Pope MD - Pediatrics & Adam Husney MD - Family Medicine & Elizabeth T. Russo MD - Internal Medicine