Tests to diagnose and monitor asthma

Alongside your symptoms and medical history, your doctor uses asthma tests to help confirm or rule out asthma. These tests can also be used to monitor your asthma.

Health advice > Diagnosing asthma

Find out about tests used to diagnose and monitor asthma, how they can help to confirm an asthma diagnosis, and why you may need to do tests more than once to get useful results.

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Why do doctors use tests for asthma?

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

Symptoms of asthma, like cough, breathlessness, and wheeze can also be caused by other conditions too. Once your doctor has looked at your symptoms, triggers, and your family and clinical history, asthma tests are the best way to confirm or rule out asthma.

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

Most asthma tests are suitable for adults, young people, and children over five. Not everyone will need to do all of these tests.

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.

They can then try doing tests again in about six months or a year. Find out more about getting a diagnosis for your child.

You may need to do tests more than once

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, or a few different tests, to help your GP make the right diagnosis.

Spirometry and bronchodilator reversibility test

Spirometry is a breathing or ‘lung function’ test. It measures how much air you can breathe out in a set time and how narrow your airways are. This helps your GP or asthma nurse work out how likely or not it is that you have asthma.

This test is suitable for adults, young people, and children over five. Spirometry is safe and quite easy to do, but some people can feel a bit dizzy or faint afterwards.

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 and as hard as you can, for as long as you can, through a mouthpiece linked to a ‘spirometer’ machine.

You will have to blow a few times so your GP or asthma nurse can get an accurate result.

Bronchodilator reversibility test

If the spirometry test shows that your airways are narrowed, you may need to do a bronchodilator reversibility (BDR) test. This involves taking some asthma reliever medicine through a spacer. Reliever medicine is a bronchodilator which means it opens up the airways.

You’ll be asked to wait 15-20 minutes and then blow into the spirometry machine a few times again. This is to see whether there’s been a change in your airways after taking the medicine. If your airways become much less narrow after the reliever medicine, it makes it 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, especially if it’s not well controlled. Your GP or asthma nurse may use it to see how well your medicines are working, particularly if your treatment plan has changed.

Your GP or asthma nurse may suggest a spirometry test at your annual asthma review.

Spirometry, alongside a BDR test, 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?

Your GP or asthma nurse will compare your results to measurements that are considered within the ‘normal’ range. This helps them 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 obstructed it could be due to inflammation. Doing spirometry again after taking the reliever medicine (the bronchodilator reversibility test) may give a different test result. The result can tell your GP or asthma nurse if your airways have responded to the medicine. A good response to the asthma medicines suggests asthma.

You may not get the results straight away. They need to be looked at by a healthcare professional who can interpret them accurately. Remember to ask when you can expect to get your results and how you will get them.

FeNO testing

FeNO Tests for Asthma

What is a FeNO test and why is it important? Dr Andy Whittamore explains what a FeNO test is and how to prepare for it.

Video: FeNO Tests for Asthma

What is a FeNO test and why is it important? Dr Andy Whittamore explains what a FeNO test is and how to prepare for it.
Transcript for "FeNO Tests for Asthma"

0:07 What is a FeNO test and why is it important? FeNO stands for Fractional exhaled Nitric Oxide. FeNO is a very safe, very effective test that can help detect allergic inflammation in your airways. Allergic inflammation is a key part of many people’s asthma and can help to us to make sure you’ve got the right diagnosis and the right treatments. FeNO is used alongside other tests, but also with a detailed clinical history. And your healthcare professional will use them, alongside FeNO, to help diagnose asthma in adults and for children over five as well.

0:45 Emma: The first time I had the FeNO test, I was slightly apprehensive, but the nurses that were there explained everything to me, made me feel relaxed beforehand, and explained that it may take one or two attempts to get the hang of it, but once you do get the hang of it it’s very simple. After the test I felt completely fine. The test itself is so mild that is doesn’t really have any problems that afterwards you need to take that recovery time. And also, I felt relieved because there was an answer to my symptoms, and I knew I was being put on the correct pathway.

1:27 How a FeNO test is done. So, the FeNO test can be performed anywhere where there is a FeNO machine. Most hospital asthma clinics have had FeNO machines for years and now GP surgeries are starting to get them as well. FeNO is a test that can be done by any healthcare professional who’s had the right training. The FeNO test takes literally one to two minutes. Once you’re settled and relaxed, you’ll be asked to take a breath in, and then you’ll breathe out into the machine which will be able to detect how much nitric oxide you’ve got in your airways. The results come up on the screen within a minute or two. Every person doing the FeNO test will use a new, clean, hygienic mouthpiece. These are disposable, so they’re not reused. Each clinic as well will have its own protocols. So that the staff, the room and the machine are all kept clean, healthy and safe for your use.

2:24 Emma: Compared to other asthma tests that I’ve had, the FeNO test is a lovely, simple breathing test. It doesn’t trigger my asthma symptoms, and it is far less hard work than a lot of the other tests are.

2:45 Understanding your FeNO test results. High FeNO level tells us that there’s some allergic inflammation in your airways. This is important because this can tell us whether you’ve got asthma, but it also tells us that your lungs are more likely to be set off by triggers, and you’re perhaps more prone to having an asthma attack as well. Really important though, is having a high FeNO level does tell us which medications work best for your lungs. Having a low FeNO level doesn’t rule out asthma. What is does though is tells us there isn’t any allergic inflammation in your airways. You might have a different cause for your symptoms, or you might have a non-allergic type of asthma that needs looking into as well.

3:24 How to prepare for a FeNO test. There are a few things we ask people not to do before having a FeNO test. So, for one hour before having the test try not to exert yourself. Try not to drink any hot drinks, especially avoiding caffeine and alcohol, and do not smoke for an hour before the test either. There are a few foods as well which can affect a FeNO test. So, for three hours before having your FeNO test try to avoid green leafy vegetables, such as celery, leek, spinach, cabbage, and also avoiding beetroot as well. Take all you medicines as normal before having a FeNO test, but do let the person doing the test know whether you’re taking some oral steroids, if there’s been any changes in your medication, or if you’ve got any illnesses at the time.

4:12 What to know more about FeNO tests? At your next routine appointment with your GP or your asthma nurse, you can always ask them whether they’ve got FeNO available yet. For more information about FeNO visit the Asthma + Lung UK website. You can also call our helpline and speak to our specialist team about whether you think FeNO would be useful for you.

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, usually 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, your 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. 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.

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 a hospital out-patients appointment for your FeNo test.

There are some things that can affect the FeNO test, like smoking, caffeine, alcohol, and green leafy vegetables. So, it is best to avoid them before the test. You should also let the GP know if you are taking, or have recently taken, steroid tablets. The NHS has guidance on how to prepare for the test.

A FeNO test is done by breathing into a mouthpiece 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.

When is it used?

FeNO can be used to help diagnose asthma in adults. It can also be used in children over five when a diagnosis is unclear.

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. 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 usually higher if you have allergic asthma.

FeNO is useful for helping your healthcare team prescribe or 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.

Once you’re taking asthma medicines, your results should show lower levels of nitric oxide in your breath compared to levels before you started treatment. This is a sign the medicines are working to control the inflammation.

The NHS has developed a leaflet to explain what FeNO is, why it is important, how the test is conducted and how to prepare for the test.

Peak Flow 

This is a lung function test to measure how fast you can breathe out. You can do this test in your GP surgery. You can also do it at home to monitor your peak flow over time.

Your GP or asthma nurse might ask you to keep a peak flow diary for two-four weeks to help confirm an asthma diagnosis. Your GP or asthma nurse will show you how to record your peak flow score and how to look after the equipment. We have a free peak flow diary to help you record your daily peak flow scores and symptoms.

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 hard and 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.

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 a 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 your GP to see how open your airways are.

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

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 results show how quickly you can breathe out and if your airways are narrowed. 

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

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.

Find out more about what your peak flow scores mean.

Bronchial challenge test

This test is only carried out in a hospital out-patients department. It measures how your airways respond to asthma triggers. It’s also 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. Guidelines recommend them for people aged 17 and over.

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 bronchial challenge test used?

This test will probably only be used if the results of other asthma tests 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 helps your doctor understand whether you have asthma.

Bronchial challenge tests are not used to monitor your asthma once you have an asthma diagnosis.

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 adults and children with asthma might have:

  • a blood test
  • a skin-prick test

How are allergy tests done?

If you have a blood test, some blood will be removed 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.

When are allergy tests used?

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

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

What do the results of an allergy test show?

A blood test will show your GP how many IgE antibodies you have in your blood. Specific tests may show how many antibodies have been produced in response to a specific allergen, like house dust mites, eggs, or cat dander.

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.

Asthma control questions

Your GP or asthma nurse may use a set of standard questions to find out about your asthma symptoms and how well they are controlled. It’s a way for your GP or asthma nurse to get specific answers to how your symptoms are now.

Questionnaires include the Asthma Control Test (ACT) and the Asthma Control Questionnaire (ACQ), and the RCP 3 Questions.

Some questionnaires are used for both adults and children aged five and over.

Others are used for just children. For example, the Child Asthma Control Test (C-ACT) for four-to eleven-year-olds, with a set of seven questions, some for the child to answer and some for the parent.

How are asthma control questions done?

Your GP or asthma nurse will ask you a few questions and give you a number score for each of your answers.

When is it used?

Your GP or asthma nurse will use the asthma control test before starting you on any trial of treatment. It’s also a good test to use when reviewing your asthma at your annual review.

What do the results show?

Each answer has a value or score. Once you’ve answered all the questions, the overall score you get shows your level of asthma control.

This shows your GP or asthma nurse any symptoms you’re having that could indicate asthma. If you have asthma already, the questionnaire shows how your asthma is, and if your treatment plan needs to change.

You can get advice and support about tests used for asthma by calling a respiratory nurse specialist on our Helpline, 0300 222 5800 (9am-5pm; Monday-Friday). Or you can WhatsApp them on 07378 606 728.


Last updated October 2021

Next review due October 2024

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