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Which measure is the single best determinant of asthma severity?

Medically reviewed by Carmen Pope, BPharm. Last updated on Dec 23, 2021.

Official answer


A peak expiratory flow rate (PEF or PEFR) is the single best determinant of asthma severity. A PEF measurement is a quick test to measure air flowing out of the lungs. During the test, you blow forcefully into the mouthpiece of a device, usually a peak flow meter (PFM), which is a portable, light, handheld device made of plastic. If the test is done in a healthcare provider's office or a hospital a much larger device, called a spirometer, is usually used. This device has a handheld mouthpiece that’s attached by a cord to a larger electronic machine.

A peak flow measurement can show the amount and rate of air that can be forcefully breathed out of the lungs after a full lung inhalation. An important part of peak flow measurement is looking at peak flow zones. These are areas of measurement that can help show early symptoms of uncontrolled asthma and are set differently for each person. There are 3 peak flow zones noted by color:

  • Green for “go.” This is 80% to 100% of your highest peak flow reading, or personal best, and is the zone you should be in everyday
  • Yellow for “caution” or “slow down.” This is 50% to 80% of your personal best and is a sign that your large airways are starting to narrow. You may have mild symptoms, such as coughing, feeling tired, feeling short of breath, or feeling like your chest is tightening.
  • Red means “stop.” This is less than 50% of your personal best and shows you have severe narrowing of your large airways. It is considered a medical emergency and you should get help right away. Your symptoms may include coughing, being very short of breath, wheezing while breathing in and out, or retractions (this is when you can see the muscles between the ribs working hard to keep you breathing). Walking and talking may be difficult.

  • Peak Flow Measurement. John Hopkins Medicine. 2021.
  • Camargo C, Rachelefsky, Schatz M. Managing Asthma Exacerbations in the Emergency Department. Summary of the National Asthma Education and Prevention Program Expert Panel Report 3 Guidelines for the Management of Asthma Exacerbations. Department of Emergency Medicine and Division of Rheumatology, Allergy, and Immunology, Department of Medicine,The%20exacerbation%20severity%20determines%20treatment.

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