Tests for asthma

Various tests are used to help diagnose and monitor asthma.

Your GP, asthma nurse or consultant will explain which tests are suitable for you or your child - some can't be used for children under a certain age.

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

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.

When is it used?

A GP 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 you to monitor and manage your own asthma. 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 - it can take some practice. They may ask you to keep a daily 'peak flow' diary to record your measurements as part of your written asthma action plan.

What do the results show?

Peak flow scores 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 score so you'll be able to see when your score goes up and down.

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

Watch a video showing you how to use a peak flow meter.


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. It's also known as the reversibility test. 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 will 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.

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 to show whether or not you have asthma, but 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 the FeNO test?

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) or a skin-prick test where your skin is pricked with a tiny amount of the allergen (a substance that causes an allergic response) 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.

What do the results show?

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.

If you have a skin prick test, your specialist will watch the area of the skin that's been pricked to see if it becomes itchy, swollen and red.

This shows your GP whether or not you have an allergy to specific allergens, such as dust mites, pollen, certain foods or pets.

If the tests come back and you're allergic to a specific thing, you can 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 August 2015