Investigations to help diagnose asthma A number of investigations can be helpful in making a diagnosis of asthma. Click on the links below to learn more.
- Spirometry
- Bronchodilator reversibility
- Fractional exhaled Nitric Oxide (FeNO)
- Peak Flow
- Challenge Tests
- Allergy Tests
- More information
Spirometry
A spirometry test measures how healthy your lungs are and can be used to help diagnose and monitor lung conditions. During the test, you will breathe out as much air as you can, as hard as you can, into a device called a spirometer. The test measures how much air you can blow out in total and how much air you can blow out in the first second of the test. You normally have to repeat the test 2 or 3 times to make sure you get an accurate reading. The test should not be painful and usually takes about 10 minutes. Asthma is likely if the amount of air blown out in 1 second is a lot different from the total amount (indicating airway narrowing) - if this is the case then a further test called bronchodilator reversibility is likely be completed (see below).
More in-depth information on spirometry is available from the European Lung Foundation
Bronchodilator reversibility
If spirometry has indicated a narrowing of the airways, a reliever inhaler (also known as a bronchodilator) is used to see if it makes the airways less narrowed. After taking the reliever inhaler you will wait for 15 or 20 minutes before repeating the spirometry test. If the reliever inhaler has improved the narrowing a lot, then asthma is confirmed.
Peak Expiratory Flow
Peak expiratory flow is a straightforward test that can be done at home and is used to measure how fast you can breathe out at a given point in time. Your peak flow result depends on your age, height, whether you're a man or a woman.
Individuals with (untreated) asthma often have a varying peak flow result over time. So, to diagnose asthma you will need to record your peak flow at least once a day for a period of time (often 2 to 4 weeks). If the amount you can breathe out varies a lot (either between morning and night, or between different days) it can indicate that you have asthma. Bear in mind that if you have already started treatment for asthma then your peak flow result is likely to show much less variation over time - so its best to do peak flow before starting treatment.
Here is a useful video showing how a peak flow test is carried out
Fractional exhaled Nitric Oxide (FeNO)
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. FeNO can also be used to help diagnose asthma in adults and children when diagnosis is unclear. The results of your FeNO test can show how inflamed your airways are and 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.
A more in-depth discussion on FeNo is available from Asthma UK
Challenge Tests
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.
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.
For more information on challenge testing see this article from Asthma UK
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 [asthma UK].
More information
You can find more information on asthma diagnosis from the NHS and Asthma UK
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