Peak expiratory flow rate

Alternative names
Peak flow


The peak expiratory flow rate measures how fast a person can exhale air. It is one of many tests that measure the function of the airways, which are commonly affected by diseases such as Asthma, or Chronic obstructive pulmonary disease (COPD, usually a combination of Emphysema and Chronic bronchitis).

In these lung diseases, air flow during exhalation is decreased by narrowing or blockage of the airways. The severity of Asthma or COPD can change with time, and peak expiratory flow monitoring is used by many patients to monitor their lung function at home. This allows them to anticipate when their breathing will become worse and to take appropriate medications or call their health care providers before symptoms become too severe.

How the test is performed
This test requires a peak expiratory flow monitor, a small hand-held device with a mouthpiece at one end and a scale with a moveable indicator (usually a small plastic arrow). Breathe in as deeply as possible. Blow into the instrument’s mouthpiece as hard and fast as possible. Do this 3 times, and record the highest flow rate.

How to prepare for the test
Loosen any tight clothing that might restrict your breathing. Sit up straight or stand while performing the tests.

How the test will feel
There is usually no discomfort. Rarely, repeated efforts may cause some lightheadedness.

Why the test is performed
Home testing may be useful in monitoring the progress of a disease such as Asthma, Emphysema, or Chronic bronchitis. It can assess the effectiveness of various treatments or detect the worsening of the condition.

Normal Values

Normal Values
vary considerably according to a person’s age, sex, and size. Peak flow measurements are most useful when a person is able to compare the peak flow obtained on a day-to-day basis. A fall in peak flow, especially when accompanied by symptoms such as increased cough, Shortness of breath, or Wheezing, may signal the onset of a flare of lung disease, requiring early treatment to prevent complications.

What abnormal results mean

If you note that your peak flow is decreasing, you should advise your healthcare provider and take any steps you and your provider have previously discussed.

What the risks are

There are no significant risks.

Special considerations

Peak expiratory flow rate measurements are not as accurate as the spirometry measurements performed in a health care provider’s office. Small changes in your peak flow may not mean significant changes in your lung function.

Johns Hopkins patient information

Last revised: December 6, 2012
by Dave R. Roger, M.D.

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