Tests to diagnose asthma

Lots of different tests are used to help diagnose and monitor asthma

Currently there’s no one test that can prove whether someone has asthma. And each test might be repeated more than once to get a full picture of your asthma.

But there are many different types of tests that your GP, asthma nurse or asthma specialist might carry out to help work out whether you or your child has asthma. These include: 

Some can also be used to monitor your asthma on a daily basis, alongside a written asthma action plan.

These tests help to show if your airways are narrow or inflamed, and how your lungs might respond to medicine. But it’s important to remember that if you get tested and your results come back normal, this doesn’t rule out a possible diagnosis of asthma. It could just mean that your airways are not as narrowed or inflamed when you get tested. Because asthma symptoms can vary over time, you may need to take these tests more than once to help your GP make the right diagnosis.  

Dr Andy Whittamore, our in-house GP, explains, ‘If you have one negative test, of all the various ones that are currently available, or you don’t have symptoms, it doesn’t mean you don’t have asthma. Asthma’s not that simple.’ Read Dr Whittamore’s blog post ‘Get the real story on asthma diagnosis’ here.

You may also be given an allergy test if your GP thinks your asthma is triggered by a specific allergen (a substance that causes an allergic response).

Your GP, asthma nurse or consultant will explain which tests are suitable for you or your child (tests for asthma are usually offered to children aged five and above). You can find out more on our page, Getting an asthma diagnosis for your child.

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

And if you have a diagnosis of severe asthma, there's a useful round-up of the things you can do to monitor the symptoms here - including the tests you might use

If you’re feeling bewildered by the results of an asthma test, or want to talk about your diagnosis with an asthma expert nurse, try our Helpline. It’s open Monday to Friday, 9 to 5: 0300 222 5800.

Peak flow

What is it?

The peak flow test (peak expiratory flow test or PEF) is a lung function test to measure how fast you can breathe out. 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.

Your GP will get you to repeat this test a few times and then they will note down your best reading.

When is it used?

A GP or asthma nurse might use a peak flow test to help see how open your airways are – they often use this test when you first tell them you're experiencing symptoms that might be asthma. Your GP or asthma nurse should do a peak flow test at every annual asthma review as part of their routine checks.

This test is also useful for monitoring your asthma using a peak flow diary alongside a symptom diary and your asthma action plan. You can buy, or get a prescription for, a peak flow meter and your GP or asthma nurse will show you how to do the test yourself at home.

What do the results show?

Peak flow readings will vary depending on your age, your height and whether you're a man or a woman. This test shows how your individual lung function changes.

Although your peak flow reading may be different in the morning or at night, it's the pattern your scores make that's important, rather than one score on its own. Most people have a best peak flow reading so you'll be able to see when your reading goes up and down.

Keeping track of your peak flow whilst monitoring your symptoms can help you spot when your asthma is getting worse and when you need to use your reliever inhaler (usually blue) or get medical help. 

Read our peak flow test page to find out more using peak flow to help you stay on top of your asthma, and warn you of an asthma attack.


What is it?

This is a breathing or ‘lung function’ test. It’s one of the most common tests people with asthma, or people who are being tested for asthma, are given. Your GP or asthma nurse will ask you to take a deep breath and then 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.

Once they have an accurate result, you may then be given a bronchodilator (reliever) medicine to open up your airways and asked to wait 15-20 minutes. You will then need to 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 they become much less narrow, it could be a sign that you have asthma.

When is it used?

This is usually the first test your GP or asthma nurse will do to help them work out how likely it is that you have asthma. For best results, it’s usually done before you start taking any preventer medicine.

What do the results show?

A spirometer measures how much air you can breathe out. By comparing your results to measurements that are considered ‘normal’, 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 ethnicity because of inflamed or narrowed airways.

They can also see whether your airways have responded to the bronchodilator medicine. If they have, it’s likely you have asthma.

This test can also help your healthcare team see if your symptoms might be caused by another lung condition, such as chronic obstructive pulmonary disease (COPD).

Airways responsiveness test

What is it?

This highly-specialised test is carried out in hospital and measures how your airways respond to asthma triggers. You’ll be asked to breathe in a substance (histamine or methacholine) that may irritate your airways. This can cause 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.

When is it used?

This test will probably only be used if the results of your spirometry test didn’t give your GP or asthma nurse a clear picture of whether or not you have asthma, and you still have unexplained symptoms.

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.

FeNO testing

What is it?

If you’re given a ‘fractional exhaled nitric oxide’ (FeNO) test, a hand-held machine is used to measure your 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 and that you have asthma.

When is it used?

If you’ve been diagnosed with asthma and you’re using your preventer inhaler correctly, but still have asthma symptoms, your GP, asthma nurse or specialist may ask you to do a  FeNO test (usually as well as a spirometry test) to help them work out the type of asthma you have.

As an alternative to this FeNO test, your specialist may take a sample of mucus (phlegm) to test for inflammation.

What do the results show?

The FeNO test checks for inflammation in your airways and tells a specialist if 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.

Allergy tests

What is it?

There are two main allergy tests people with asthma might have: a blood test (known as a ‘specific IgE’ or ‘RAST’ test) and a skin-prick test where your skin is pricked with a tiny amount of the allergen you think you might be allergic to.

When is it used?

If your GP thinks your asthma symptoms may be triggered by certain allergens, such as dust mites, mould, pollen or certain foods, you may be referred to a specialist for allergy tests. An allergy 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 well.  

What do the results show?

A skin prick test is most commonly used allergy test. Your specialist will prick your skin with tiny amounts of the suspected allergen(s) and 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.

If you have a blood test the results 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. It might take a couple of days to get the results of your blood test. 

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

Last updated November 2016

Next review due November 2019