Peak flow readings measure peak expiratory flow (peak flow) rates. This measurement looks at how well your airways are working.

Since peak flow rates can change easily, they are not used alone to diagnose asthma. In some situations, though, peak flow readings may be used when spirometry is normal and asthma is still suspected.

For diagnostic purposes, peak flow testing can be used to suggest — not diagnose directly — asthma, if all other testing has not proved positive. If you have a 20% or more change in peak flow measurements taken after waking up compared to afternoon readings collected after using asthma medicine, this would suggest asthma. This is known as variability.

If you have moderate or severe asthma, using a peak flow meter is an important part of managing your disease. From day to day, you will be able to follow the changes in your airways and tell when there is narrowing before you even feel asthma symptoms. If you use your peak flow meter to monitor your asthma every day, you will find that peak flow rates change throughout the day. Compare your readings to your personal best.

Consistently high peak flow readings tell you that you are able to manage your asthma. If you find that your peak flow readings vary a lot in a day, this could be a sign that your asthma is not well controlled. The lowest peak flow readings are typically in the morning, between 4 A.M. and 8 A.M. The highest readings tend to be midday and in the late afternoon, between 4 P.M. and 6 P.M.

Peak flow is interpreted in color zones, similar to a traffic light. Green, yellow, and red zones are used to guide you in action steps for managing asthma. The action steps that correspond with each of these zones will be written in your Asthma Action Plan. Your peak flow meter reading will fall into one of these three zones:

Green Zone: "Go"
  • at least 80% of your personal best
  • Your asthma is well controlled.
Yellow Zone: "Caution"
  • 50% to less than 80% of your personal best
  • Your asthma is getting worse.
  • Asthma symptoms might start acting up.
  • Monitor your peak flow frequently.
  • Follow instructions outlined in your Asthma Action Plan as recommended by your doctor. You might need to increase or change medicine, reduce your exposure to asthma triggers, or change another part of your plan.
Red Zone: "Danger"
  • less than 50% of your personal best
  • Your asthma is in the danger zone.
  • Stop what you are doing and immediately follow the Asthma Action Plan steps recommended by your doctor.
  • Next, call your doctor.
  • If there is no improvement and you are still in the red zone after 15 minutes and have not reached your doctor, go to the hospital or call for an ambulance.

Watch for danger signs and act. If you have trouble walking and talking due to shortness of breath, or if your lips or fingernails are blue, take your quick-relief medicine and immediately go to the hospital or call for an ambulance.

Written by Karen Serrano, MD

Emergency Medicine resident at the University of Wisconsin-Madison.

Reviewed by Lisa V. Suffian, MD

Instructor of Clinical Pediatrics in the Division of Allergy and Pulmonary Medicine at Saint Louis Children's Hospital, Washington University School of Medicine

Assistant Clinical Professor in the Department of Pediatrics at Cardinal Glennon Children's Hospital, Saint Louis University

Board certified in Allergy and Immunology

Last updated June 2008