The peak expiratory flow rate measures how fast a person can breathe out (exhale) air. It is one of many tests that measure how well your airways work.

This test requires a peak expiratory flow monitor: a small handheld device with a mouthpiece at one end and a scale with a moveable indicator (usually a small plastic arrow).

To do this test:

• 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.

Loosen any tight clothing that might restrict your breathing. Sit up straight or stand while performing the tests.

There is usually no discomfort. Rarely, repeated efforts may cause some light-headedness.

The test is commonly used to diagnose and monitor lung diseases such as:

• Asthma
• Chronic bronchitis
• Chronic obstructive pulmonary disease (COPD)
• Emphysema

Home monitoring can help determine whether treatments are working or detect when your condition is getting worse.

Normal values can vary based on a person's age, sex, and size. Peak flow measurements are most useful when a person is able to take and compare peak flow measurements on a day-to-day basis.

A fall in peak flow can signal the onset of a flare of lung disease, especially when accompanied by symptoms such as:

• Increased cough
• Shortness of breath
• Wheezing

This may require early treatment to prevent complications.

Air flow during exhalation decreases when the airways are narrowed or blocked.

Many patients can use peak expiratory flow monitoring to monitor their lung function at home. This allows them to anticipate when their breathing will become worse and to take medications or call their health care providers before their symptoms become too severe.

If you note that your peak flow is decreasing, you should tell your health care provider.

There are no significant risks.

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.