Peak flow test

Peak flow is a measure of how quickly you can blow air out of your lungs. If you have asthma, your airways can sometimes become inflamed or narrowed. This can make it more difficult to blow the air out of your lungs quickly and can change your peak flow score.

You measure peak flow using a small hand-held device called a peak flow meter. See our instructions for using a peak flow meter, including a video explanation.  

Your peak flow score is also called your peak expiratory flow (PEF).

Peak flow is both used to diagnose asthma and to help monitor it. The information on this page is mainly about helping you understand how to use peak flow to help monitor your asthma.

Peak flow to help diagnose asthma

If you’re having symptoms that could be asthma, your GP may ask you to keep a peak flow diary for two or more weeks to help them make a diagnosis. The GP will compare your pattern of peak flow scores to what’s expected for your age, height and sex.  

If there’s a big difference between your score and the expected score, or your score when you have symptoms or not, it could suggest you have asthma. 

Your GP will consider your general health and the results of other tests, like spirometry and FeNO, along with your peak flow scores to help decide whether you may have asthma.

Peak flow to monitor asthma 

Some people find that peak flow monitoring helps them to notice early signs that their asthma may be getting worse.    

Healthcare professionals do not always recommend peak flow monitoring for asthma. This is because for some people, understanding how their asthma symptoms change is the best way to manage their asthma well, rather than focusing on peak flow scores.  

The most important thing is to monitor your asthma in a way that works well for you. You could use our symptom diary to record your day-to-day symptoms.  Understanding your usual symptoms could help you to notice any patterns or changes in your symptoms that may lead to an asthma attack. 

Talk to your GP or asthma nurse about whether monitoring your peak flow would be helpful for you. They should explain how often to check it, and what to do when your scores change. Make sure this is written in your asthma action plan.

Your healthcare professional may also do a peak flow test at your annual asthma review.

There are times when you might be asked to check your peak flow at home. This means taking your peak flow every day and writing down your scores in a peak flow diary. 

Checking your peak flow at home is most useful if you’re recovering from an asthma attack, your treatment plan has changed, or your symptoms are difficult to control.

When you first get a peak flow meter, ask your GP, asthma nurse or pharmacist to show you how to use it and how to take a reading.

Take these steps to make sure you use your peak flow meter correctly: 

1. Pull the counter (the red arrow) back to where the numbers start. The counter is sometimes called a pointer.
2. Stand or sit upright – choose what’s most comfortable for you and always do it that way.
3. Take the deepest breath in you can.
4. Make sure your mouth makes a tight seal around the mouthpiece.
5. Blow as hard and fast as you possibly can into the meter.
6. Write down your score – this is the number next to the pointer.
7. Do steps 1 to 6 three times in a row, with a short rest in between, so you get three scores. All three scores should be roughly the same.
8. Use the highest of these scores to fill in your peak flow diary.

As well as the pointer that shows your peak flow score, some peak flow meters may have more coloured markers on the side. These may be red, amber, and green. Ask your healthcare professional to set these to show your best and worse peak flow scores. This can help you quickly see what your current score means and if you need to take any action from your asthma action plan.

If you’re using peak flow to help monitor your asthma, the main thing is knowing the score that’s normal for you, sometimes called your personal best score. This can help you notice if your peak flow score drops lower than usual.

Keep a peak flow diary

To understand your own peak flow, you need to see how your scores change over time. This means it’s important to note down your scores every day for as long as your healthcare professional has asked you to. 

You can do this using a peak flow diary. Or you might want to note your scores on your phone using the calendar or a peak flow diary app. Many peak flow meters come with a peak flow calendar to note your scores. 

You and your GP or asthma nurse can then look at the scores you’ve been getting and work out what score to expect when you’re well. Your asthma action plan can help you manage your asthma and recognise when your symptoms are getting worse.

Your best score

The score that’s usual for you when you’re well is sometimes called your ‘best score’.

If you’re using your medicines as prescribed and following your asthma action plan, you should stay close to your personal best score most of the time. 

If you’ve recently changed your treatment plan and your peak flow score improves, this may help show you that your treatment is working well.

You need to record your peak flow for at least two weeks to find your personal best score. Ask your GP to check it again if it was last checked a few years ago, or if you change your peak flow device. 

Lower than your best score 

Talk to your GP or asthma nurse about what low scores you need to look out for, and what action you need to take. Write this down in your asthma action plan.

If you get a peak flow score that’s lower than your best score, this may be an early sign of an asthma attack or flare-up. 

Getting a lower peak flow score can also help you find out what triggers could be making your asthma worse. For example, you might notice lower peak flow readings when pollen’s high, you’ve got a cold, or when you’re at work. 

How much daily change is normal?

The scores you get, and any changes from them, are personal to you. Your GP or asthma nurse will help you understand what changes in your peak flow score you need to look out for and what to do if your scores change. 

Make sure this is written in your asthma action plan.

It’s normal to see slight differences in your peak flow score, either between morning and evening scores, or from one day to the next. For example, people often have a lower score in the morning. 

What if I get a much higher or lower score than usual?

You may sometimes get a peak flow score that’s much higher or lower than usual. A sudden drop in your peak flow score can be a sign that your asthma is getting worse, especially if your symptoms are getting worse too. 

There can be other reasons you get a much higher or lower score, for example: 

• how you’re using your peak flow meter  – follow our steps on how to use your peak flow meter in the best way
• your peak flow meter may need cleaning or replacing – check the information leaflet that comes with your device.
• if you used a different peak flow meter than usual. Sometimes, a healthcare professional may also take your reading on a different peak flow device. Remember to set a new personal best reading with your healthcare professional if you get a new meter.

Follow our advice above to make sure you get the most accurate results.

Always check with your GP, asthma nurse or pharmacist if you have any questions.

This graph shows the peak flow scores of people without a lung condition.

It shows that peak flow scores are usually expected to get lower with age. This is because it’s natural to have a gradual decrease in how well your lungs work as you get older. Your healthcare professional will help you understand your peak flow results and what they mean for you.
 

You can get a peak flow meter on prescription from your GP or asthma nurse. If you pay for your prescriptions, you’ll need to pay the usual cost for one peak flow meter.

You can also buy a peak flow meter yourself from a pharmacy or online. 

If you’re buying one for a child, make sure it has a smaller mouthpiece and a lower scale.