The peak expiratory flow rate measures how fast a person can breathe out (exhale) air. It is one of many tests that measures how well the lungs are working.

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

• Asthma
• Chronic obstructive pulmonary disease (COPD)
• Rejection after a lung transplant

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

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 perform this test:

• Breathe in as deeply as possible.
• Blow into the instrument's mouthpiece as hard and fast as possible.
• Do this three 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.