Peak flow test

Noting your peak flow reading every day can help you stay well with your asthma and reduce your risk of an asthma attack

Peak flow in brief

  • The peak flow test (peak expiratory flow test or PEF) measures how fast you can breathe out, so you can see how well your lungs are working. 
  • Peak flow is one of the few ways you can keep a close eye on your asthma and have a better understanding of what’s going on in your lungs.
  • Peak flow is also used to help diagnose asthma. If you’re having symptoms that might be asthma, your GP will measure your peak flow to see how open your airways are.
  • To measure your peak flow, you take a deep breath in and blow as fast as you can into a small, hand-held plastic tube called a peak flow meter. The measurement taken is called your peak flow.
  • To get the best from your peak flow meter, you need to take your readings every day, twice a day, and keep a diary of symptoms. This is so you can see when your medicines are working and spot when your asthma is getting worse so you can take action and reduce your risk of an asthma attack.   

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Why it's useful to know your peak flow reading

Measuring your peak flow, alongside keeping a note of your asthma symptoms, is a simple but useful way to help manage your asthma better, as your pattern of readings will tell you if your lungs are working as well as they should be.

If your airways are open and working well, you should be able to blow into your peak flow meter quickly and forcefully - and your peak flow reading should be high. If your airways are tight and inflamed, you won’t be able to blow into the meter as quickly, and your peak flow reading will be low. This could be a sign that your airways are struggling and that your asthma is getting worse.

Whether you’re recording your peak flow reading every day, or for a set period of time as advised by your GP or asthma nurse, using a peak flow diary can help you know what reading to expect when your asthma is well managed, and what reading may mean you need to take action to reduce your risk of an asthma attack. It’s important to know that your peak flow readings will be personal to you, as they vary depending on height, age and gender. 

By recording your peak flow, you can:

  • Keep track of your peak flow in between your asthma reviews
  • Identify and record any triggers or allergies that could be making your asthma worse
  • See how much of a difference you can make to your airways by taking your asthma medicines every day
  • Spot the early warning signs that your airways are struggling so you can take quick action to avoid an asthma attack
  • Monitor your asthma after an asthma attack

"Right from the time I was diagnosed, I've taken my peak flow measurement twice a day to help me manage my asthma. This helps me track how well I'm doing. I like to think that I'm managing my symptoms with the support of my GP, rather than expecting her to manage them." - Vicki Shenton 

How to use a peak flow meter

If you have a peak flow meter, take these steps to make sure you use it correctly:

  1. Get yourself into a comfortable position, either sitting upright or standing (choose what’s easiest for you and always do it in that way).
  2. Reset your peak flow meter so the pointer is pushed back to the first line on the scale. This is usually 60.
  3. Hold the meter so it’s horizontal and make sure your hands are not blocking the measurement scale. 
  4. Take a deep breath and fill your lungs all the way.
  5. Place the meter to your mouth and make sure you’re your lips form a tight seal around the mouthpiece. Do not put your tongue inside the hole.
  6. Blow as hard and fast as you can into the meter.
  7. Write down your reading (the number displayed next to the pointer).

Repeat these steps three times and make note of your peak flow reading each time. All three results should be roughly the same, but it’s the highest reading you need to write in your peak flow diary.

How to take your peak flow reading

Asthma UK nurse, Suzanne, demonstrates how to use your peak flow meter to get a reading.

Video: How to take your peak flow reading

Asthma UK nurse, Suzanne, demonstrates how to use your peak flow meter to get a reading.
Transcript for 'How to take your peak flow reading'

0:05 To take your peak flow reading make sure you’re either sitting in a chair or standing up

0:10 but always do one or the other.

0:13 Your peak flow meter has a small arrow and you need to push this right to the top near the mouthpiece.

0:21 Then hold your peak flow meter like so, so your fingers are not going over the arrow.

0:26 Take a deep breath in and then you blow hard and fast into the mouthpiece like this.

0:35 Wherever the arrow goes, next to which number, that would be your peak flow

0:38 score which you can then plot in your peak flow diary.

Related resources
Peak Flow Diary

Peak Flow Diary

Download a diary to track your peak flow.

Download (1 MB)
Adult Written Asthma Action Plan

Adult action plan

Download your asthma action plan to fill out with your GP or nurse here.

Download (675 KB)

“If you’re finding it difficult taking your peak flow, or if you feel breathless or tight-chested afterwards, speak to your GP or asthma nurse, or call the Asthma UK Helpline for advice on 0300 222 5800 (Mon – Fri; 9am-5pm)” says Asthma UK’s head of helpline, Sonia Munde.

You can also email the Helpline directly with your peak flow questions by using our Helpline Contact Form

Making the most of your peak flow

Measuring peak flow is another way you can manage your asthma well, but for your readings to be really useful, you need to:

  • Take your peak flow twice a day, every day to get a useful pattern of results.
  • Take it at the same time every morning and every evening as your peak flow can vary throughout the day. 
  • Take it before you take your preventer inhaler otherwise the medicine could change your peak flow reading.
  • Use your best effort each time you blow into the meter so you’re comparing like with like.
  • Use the same peak flow meter each time. 

Knowing your peak flow readings

Peak flow readings vary depending on your gender, age and height. They will also vary at different times of the day, which is why you should always take your peak flow both morning and night.

Your peak flow readings can help with an asthma diagnosis

If your peak flow is being measured to help diagnose whether or not you have asthma, your GP will compare your peak flow reading to what would usually be expected of someone your age, gender and height. 

Your peak flow readings can help you to manage your asthma

Recording your daily peak flow readings is another way to keep an eye on your asthma and how healthy your lungs are. Each time you use your peak flow meter, make sure you take your reading three times and use the highest as your final result. You can compare your highest reading with your ‘personal best’ reading - the reading that is usual for you when your asthma is well controlled.

If you’re taking your asthma medicines as prescribed and following your asthma action plan, your peak flow reading should be close to your personal best most of the time.

Knowing your personal best peak flow reading means you can tell if your asthma medicines are working well, and can quickly spot any signs that your asthma is getting worse. This is because you’ll have a clear idea of what a ‘good’ reading looks like, so if at any time you get a low reading after taking your peak flow test, you will know straight away that you need to take action to get your asthma back under control.

A guide to peak flowHow to find your best peak flow reading

If you’ve just started taking your peak flow, your GP or asthma nurse can help you find your personal best at a time when your asthma is well controlled. They will encourage you to keep a peak flow diary for a specific period of time, so they can use this to work out what your personal best peak flow reading is - based on your height, gender and age. 

It’s important that you work with your GP or asthma nurse to update your best peak flow reading every few years, and more often for children. This will help you to feel confident that you’re managing asthma well over time. 

What a low peak flow reading means

There may be times when your peak flow reading is lower than your personal best. This could be a sign that you need to take action to stop your asthma getting worse. Talk to your GP or asthma nurse about what low readings you need to look out for. They will use your personal best to work out what readings you need to look out for that show you’re at risk and need to get help.

If your peak flow reading falls below your personal best, refer to your written asthma action plan which tells you what to do. You may need to use your reliever inhaler (usually blue) to provide on-the-spot relief from asthma symptoms.

But if your symptoms are getting worse and/or you’re using your reliever inhaler more than three times a week, you need to book an urgent appointment with your GP or asthma nurse within 24 hours. They will help you take action to ward off an asthma attack by reviewing your asthma medicines.

You may be prescribed a short course of oral steroids to help get your symptoms back under control. 

What it means when your peak flow is good, but you’re getting asthma symptoms

There may be times when you’re not feeling completely well with your asthma but your peak flow reading is close to, or higher, than your personal best. You might be getting asthma symptoms such wheeziness or breathlessness, but your peak flow tells you that your lungs are working fine and your asthma is under control. If this happens to you, it’s important that you don’t ignore symptoms. Check your asthma action plan so you know what to do to reduce your risk of an asthma attack. 

Ultimately, peak flow is just one of the ways in which you can keep an eye on your asthma, and it should be used alongside other tools such as a written asthma action plan. Don’t forget to record your symptoms alongside your peak flow and jot down what you have been doing during the week so you can get an overall picture of your asthma and spot any changes, such as the start of symptoms, so you can get help quickly. 

Peak flow good habits

  • Get into a good routine. If you can, try to take your peak flow every day, twice a day. You might find it helps you stick to a good routine if you write down exactly when you’re going to take your peak flow so you can stick to it. Keeping your peak flow meter on your bedside table will also help you to remember to take your peak flow as soon as you wake up in the morning and at night before you go to bed. 
  • Record your peak flow readings in a peak flow diary so you can monitor patterns.You’ll get a clearer picture of when your asthma medicines are working, and be able to spot if your asthma is getting worse and when you need to take action.
  • Use a peak flow diary alongside a written asthma action plan so you can be confident that you’re on top of your asthma symptoms and know what to do if your peak flow reading drops below a certain level. You can write this in your action plan.
  • Take your peak flow before you use your preventer inhaler
  • Record symptoms alongside your peak flow to keep an eye on your asthma. If your symptoms change but your peak flow reading is the same, you still need to book an appointment with your GP or asthma nurse.
  • At your asthma review ask your GP or asthma nurse to check that you’re taking your peak flow reading correctly and getting an accurate reading.
  • Always use the same peak flow meter as different peak flow meters might give different readings, and this could affect your reading.
  • Clean your peak flow meter regularly by soaking the mouthpiece and tube in warm water mixed with a mild detergent. Finish by rinsing in clean water and shake gently to remove any excess water. Don’t scrub the inside the tube as this could damage it. You should clean your peak flow meter at least once a month to keep it in good condition. 
  • Store your peak flow meter somewhere safe so it doesn’t get damaged. Safe places to keep your peak flow meter include the drawer of your bedside table, in your bathroom medicine cabinet, or in a special pouch or case.  

Last updated October 2016