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Breathing and lung function tests

Peak flow test

What is a peak flow test?

The peak flow test measures how fast you can breathe out after you’ve taken a full breath in. Your peak flow score is sometimes called your peak expiratory flow (PEF).

A peak flow test

What’s it used for?

Asthma and peak flow

Your GP or nurse should ask you to do a peak flow test at your annual asthma review. You may also be asked to monitor your own peak flow at home regularly, as part of your asthma action plan. These results are kept in a peak flow diary to see if your peak flow varies. This can be a feature of asthma, especially if it is not under control.

What happens during a peak flow test?

You take the biggest breath in that you can. Then blow out as fast as you can, into a small, hand-held plastic tube called a peak flow meter. You don’t need to empty the lungs completely – just a short, sharp blow, as if you’re blowing out a candle. The measurement taken is called your peak flow.

Each time you check your peak flow, you should do 3 blows, with a short rest in between the blows. The best of the 3 is the one that should be recorded.

Your health care professional will make sure that your technique is correct, as this may affect the readings.

What will the results look like?

Peak flow scores will vary depending on your age, your height and whether you’re a man or a woman. The expected values are higher in younger people, taller people and men.

Peak expiratory flow (PEF) is measured in litres per minute. Normal adult peak flow scores range between around 400 and 700 litres per minute, although scores in older women can be lower and still be normal. The most important thing is whether your score is normal for you. Health care professionals will be looking to compare your scores over time, to see if your results are going up or down.

Your peak flow reading may vary through the day and night. The amount of variation is important as well as the pattern.

Keeping track of your peak flow can help you spot when your symptoms are getting worse and when you need to take your reliever inhaler or get medical help.

Range of normal values for a peak flow test

Last updated: Thursday 17 September 2020

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Last medically reviewed: January 2020. Due for review: January 2023

This information uses the best available medical evidence and was produced with the support of people living with lung conditions. Find out how we produce our information. If you’d like to see our references get in touch.