Tests to diagnose and monitor asthma

You may have to do different tests, and do some tests more than once, to help diagnose asthma. Some tests are also used to keep an eye on your asthma.

How is asthma diagnosed?

Asthma nurse Sue explains how asthma is diagnosed by a nurse or GP - including the types of asthma tests that are used

Video: How is asthma diagnosed?

Asthma nurse Sue explains how asthma is diagnosed by a nurse or GP - including the types of asthma tests that are used
Transcript for 'How is asthma diagnosed?'

0:00 If you or your GP suspect that you may have asthma, they will start off by making sure they've got an accurate

0:07 history of your symptoms, your lifestyle, any past medical history, and also family history - to see if asthma is a

0:16 possibility. To help with diagnosing asthma, it's important to do some tests, to find out exactly how well your lungs are working, and to be able to identify

0:27 whether they're inflamed, which is a key aspect of asthma. One of the tests you'll be asked to do is to blow into a peak flow meter. This is a child's version.

0:37 It's very difficult to diagnose asthma in children, but once they're about 5 to 6 years of age they may be able to blow into one of these. Blowing into one of

0:47 these will give us a reading of how fast you can blow your air out. You may be asked to take this home with you and to record peak flow readings over several

0:59 times a day to get a variation, and that will help the doctor actually diagnose whether you've got asthma or not. You'll also be asked to do a spirometry test.

1:09 This measures lung function; how much your lungs actually can hold, and how quick you can blow your air out. Now this takes a little bit more time, and you would have to attend the surgery to have

1:22 this done. You may be at the surgery between 20 minutes and an hour depending on the test. You'll be asked to blow into this spirometer. After the test you will have four puffs of a blue reliever

1:37 inhaler, through the spacer, and after 20 minutes you'll be expected to do the test again. And these results will show us, compared with your age and your

1:48 height, whether they are normal or less than predicted. It's important to remember that you may have to do these tests several times to ensure that we

2:01 get an accurate diagnosis. Asthma can vary day to day, week to week, month to month, so you may get an abnormal test one day, and it might be normal

2:13 another. Even if all the results from the tests come back as normal it doesn't exclude asthma, and your GP may decide to put you on a trial of treatment which

2:24 you would have to take for about six, eight weeks and see all your symptoms got better

“Asthma symptoms can vary over time. So, if one test comes back normal it doesn’t completely rule out asthma. It could just mean that on the day of the test your airways were not as narrow and inflamed. You may need to do some tests more than once to help your GP make the right diagnosis.”  Dr Andy Whittamore, Asthma UK’s in-house GP.

Asthma tests help diagnose and monitor asthma. They can also show how your lungs respond to asthma medicine.

Once your doctor has looked at your symptoms and what sets them off, and at your clinical history, asthma tests are the best way to confirm or rule out asthma.

Test to diagnose and monitor asthma are usually only suitable for children aged five and above. Find out more about getting a diagnosis for your child.  

Most tests are suitable for children over five. However, some children over five may not be able to do the tests well enough yet. If so, your GP will continue to treat their symptoms and keep an eye on them. Then try doing tests again in about six months or a year.

As well as the tests listed here, there are some other tests which asthma specialists might use to help diagnose and monitor severe asthma.

Peak Flow 

This is a lung function test to measure how fast you can breathe out. You can do this test in the GP surgery.

You can also do it at home to monitor your peak flow over time. Your GP or asthma nurse will show you how to do it, and how to look after the equipment.

Your GP or asthma nurse might ask you to keep a peak flow diary for 2-4 weeks to help confirm an asthma diagnosis.

Peak flow is suitable for adults and children over five, as long as they’re able to blow well into the device.

How is a peak flow test done?

You take a full breath in, then blow out 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’. It is measured in litres per minute.

You need to repeat this test a few times so your GP or asthma nurse can note down your best reading out of a few readings.

It’s useful to know your personal best reading because when you do peak flow tests in the future they can be compared to your personal best reading.

When is peak flow test used?

You might be asked to do a peak flow test when you first go to the doctors with symptoms that might be asthma. It helps the doctor to see how open your airways are.

Sometimes if other tests such as FeNO and Spirometry haven’t given certain results, your GP will want to monitor your peak flow for 2-4 weeks. But peak flow readings alone are not a reliable way to confirm or rule out asthma in children.

Peak flow is also used once you have an asthma diagnosis to check how your airways are. You can take your own peak flow and record the readings in a peak flow diary.

Your GP or asthma nurse might do a peak flow test at your annual asthma review as part of their routine checks. They will compare the result against your best reading. They can also look at the peak flow diary you’ve been keeping at home.

What do the results show?

Your peak flow result shows how well your lungs are working. 

Scores will vary depending on your age, your height, and whether you're a man or a woman. They can also be different depending on whether you do your peak flow in the morning or at night.

Dr Andy Whittamore, Asthma UK’s in-house GP says: “It's the pattern your scores make that's most important, rather than one score on its own. Most people have a ‘best’ peak flow score so you'll be able to see when your score goes up and down.”

Keeping a peak flow diary, alongside tracking your symptoms, can help you to work out your asthma triggers, and to see when your asthma is worse. It can help you stay on top of your asthma.

If your peak flow scores are lower than usual, and you’ve noticed more symptoms, it’s a sign that you need to see your GP, or get medical help, before things develop into a possible asthma attack. 


This is a breathing or ‘lung function’ test. It measures how much air you can breathe out in a set time. 

Spirometry is usually the first test your GP or asthma nurse will do to help them work out how likely or not it is that you have asthma.

This test is suitable for adults, young people and children over 5.

How is a spirometry test done?

You may be asked to do a relaxed breath first - it is often described as a big sigh into the machine.

Your GP or asthma nurse will then ask you to take a deep breath and breathe out as fast as you can, and for as long as you can, through a mouthpiece linked to a ‘spirometer’.

“You will have to blow a few times so your GP or asthma nurse can get an accurate result. It is quite hard work,” says Debby Waddell, asthma nurse expert.

Once there’s an accurate result, you may be asked to take some asthma reliever medicine through a spacer. This is to open up your airways. This is known as bronchodilator reversibility (BDR).

You’ll be asked to wait 15-20 minutes and then blow into the machine a few times again.This is to see whether there’s a big change in your airways after taking the medicine. If your airways become much less narrow, it makes it much more likely that you have asthma.

When is a spirometry test used?

Spirometry is used to compare how narrow your airways are before and after using reliever medicine. It is used by your GP or asthma nurse to help diagnose asthma.

If you already have asthma, spirometry is also used to monitor your asthma. Your GP or asthma nurse can use it to see how well your lungs are functioning, and how well your medicines are working.

Your GP may suggest a spirometry test at your annual asthma review. This is usually done in a separate appointment by a nurse or by a qualified healthcare assistant.

You may not get the results straight away. They need to be looked at by someone who knows more about asthma who can interpret them accurately. How long it takes can vary so remember to ask when you can expect to get results, and how you will get them.

Spirometry can also be used to see if symptoms might be caused by another lung condition, such as chronic obstructive pulmonary disease (COPD), or Asthma/COPD overlap syndrome (ACOS). ACOS is where people have features of both asthma and COPD.

What do the results show?

By comparing your results to measurements that are considered ‘normal’ range your GP or asthma nurse can work out if the amount of air you’re able to breathe out is less than the expected range for your age, height and gender.

The test shows if your airways are narrow or restricted. Spirometry doesn’t measure how inflamed your airways are. But if the results show your airways are blocked it could be due to inflammation.

Doing the test again after taking the reliever medicine may give a different test result. The result can tell your GP if your airways have responded to the medicine. A good, strong response to the asthma medicines suggests asthma.

FeNO testing

FeNO stands for ‘fractional exhaled nitric oxide’. A FeNO test measures the levels of nitric oxide when you breathe out.

Nitric oxide is produced in your lungs when your airways are inflamed because you’re allergic to something you’ve breathed in.

A high level of nitric oxide in the air you breathe out can be a sign that you have inflamed airways. Alongside other tests, clinical history and a trial of treatment, a  FeNO test can help confirm an asthma diagnosis.  

This test is suitable for adults and children over five. But some younger children over five may still not be able to do the test well enough.

How is a FeNO test done?

FeNO testing may be done in your GP surgery if the equipment is available. Or you may need to go to hospital out-patients appointment for your FeNo test.

A FeNO test is done by breathing into a tube attached to a hand-held monitor. The reading shows up on the monitor.

First you need to breathe in deeply, with your mouth open. Then you’ll be asked to breathe out slowly and steadily into the mouthpiece. The test is very quick and easy to do.

You may need to do the test a few times to get an accurate result.

This test can be used for adults and children over 5.

When is it used?

FeNO can also be used to help diagnose asthma in adults and children when diagnosis is unclear. It can help work out what type of asthma you have (allergic or non-allergic).   

This is because levels of nitric oxide in your breath are higher if you have allergic type asthma.

FeNO is not usually used in routine appointments to monitor your asthma. But it may be used in specialist clinics if your specialist thinks it's useful and works for you.

What do the results show?

The results of your FeNO test can show how inflamed your airways are. A FeNo test can also show whether you have allergic, or non-allergic, asthma.  

This is useful for helping your specialist to adjust the dose of your medicine or change your treatment to help you stay well with asthma. For example, if the FeNO test shows your airways are inflamed this can be controlled with asthma medicines.

This will then show up as lower levels of nitric oxide in the breath.

There are some factors that can influence the results of your FeNO test. For example, if you’re a smoker.

This is why FeNO is often done in conjunction with other tests, or a trial of treatment, in order to get the best all round picture of your symptoms, and a clear diagnosis.

Bronchial challenge test

This highly-specialised test is only carried out in a hospital out-patients department. It measures how your airways respond to asthma triggers. This is known as a direct challenge test.

The idea is to deliberately irritate your airways to test the response. After the challenge test you’ll be asked to do a spirometry test. This is to see how sensitive your airways are.

This test is not usually recommended for children.

How is a bronchial challenge test done?

You’ll be asked to breathe in a substance (histamine or methacholine) through a hand-held device or a nebuliser.

The substance causes the airways to become inflamed and narrower and trigger asthma symptoms.

Don’t worry – if any asthma symptoms are triggered, your specialist will be on hand to reassure, monitor and treat you if necessary.

After the test you’ll be given reliever medicine to open the airways again.

When is a challenge test used?

This test will probably only be used if the results of other tests, including spirometry, didn’t give a clear picture to show whether you have asthma, but you still have unexplained symptoms.

It is used to measure how your airways respond to triggers. This gives the doctor a clue about whether you have asthma.

Challenge tests are not routinely used for children. Guidelines recommend them for adults seventeen and over. They are not used to monitor your asthma control once you have asthma.

What do the results show?

Your asthma specialist can use the results of this test to work out if and how your airways react to substances that can trigger asthma symptoms.

They can then use this information to decide which asthma treatments are likely to work for you.

If you don’t have a reaction to the substances you breathe in, it’s less likely you have asthma. But if the results show your airways reacted to the substances you breathed in, it could mean you have asthma.

Allergy tests

There are two main allergy tests people with asthma might have:

  • a blood test
  • skin-prick test

Allergy tests are not used to diagnose asthma. But if your blood test, or your skin prick test, come back positive for having allergies, it could mean you’re more likely to have asthma.

Allergy tests are mostly used once you’ve been diagnosed with asthma. They identify any allergies which could be setting off your asthma symptoms.  

How are allergy tests done?

  • If you have a blood test, some blood will be removed, usually in a hospital out-patients department, and tested to see the level of antibodies. The number of antibodies will show how allergic you are in general. You should get the results back from your GP in a few days.
  • If you have a skin prick test, this is usually done in a hospital out-patients department. A specialist will prick your skin with tiny amounts of the allergen you think you’re allergic to and will look for signs that your skin becomes itchy, swollen and red. You should get the results of your skin prick allergy test within about 20 minutes.

What do the results of an allergy test show?

  • A blood test will show your GP how many IgE antibodies in your blood have been produced by your immune system in response to a suspected allergen.The results may show that your asthma is triggered by many different types of allergens – some you may not have been aware of. Or they may show that you’re allergic to one specific thing.
  • The results from a skin prick test will be able to pinpoint exactly what you’re allergic to so you can put the right steps in place to manage your asthma.

If the tests come back and you’re allergic to a specific thing, talk to your GP or asthma nurse about how to manage it, or, if possible, how to avoid it.

For example, if you’re allergic to a certain type of tree pollen, you can talk to your GP, asthma nurse or pharmacist about which hay fever medicines might be suitable for you to take during the months that pollen is in season.

More support and information about asthma tests

If you’re not sure what the results of an asthma test mean, or you want to talk to one of our asthma nurses about an asthma diagnosis, you can call our Helpline on 0300 222 5800, Mon-Fri, 9-5pm.

Or you can chat with our asthma nurses via WhatsApp  on 07378 606 728


Last updated July 2018

Next review due July 2021

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