Diagnosis

Diagnosing asthma

Asthma is usually diagnosed based on your symptoms and a few simple tests. In most cases, a GP can make the diagnosis, but if there is any uncertainty, they may refer you to a specialist for further assessment.

Tests to measure lung function

You may be given lung function tests to assess how much air moves in and out of your lungs as you breathe. These tests include spirometry and peak flow measurements.¹ Another type of breath test, known as the fractional exhaled nitric oxide (FeNO) test, can also be used to assess lung inflammation, although it is currently less widely available.^1,2

three boxes describing tests - Spirometry, Peak flow, and FeNO test. Each box describes what the test measures and what they are looking for. First is Spirometry, “Measures: how much air you can exhale after a deep breath and how fast you can breathe out. Looking for: narrowing of your bronchial tubes (tubes that let air in and out of your lungs, allowing you to breathe)”. Second is Peak flow, “Measures: how hard you can breathe out, Looking for: lower than usual peak flow reading are a sign that your lungs may not be working as well and that your asthma may be getting worse *if you have low peak flow readings, your doctor should advise you on how to track and manage this.” Third is FeNO test, “Measures: Level of nitric oxide (NO) in your breath, Looking for: Signs of inflammation in your lungs. When the airways are inflamed (a sign of asthma), you may have higher NO levels.”

Lung function tests are often done before and after taking a medication called a bronchodilator (a drug that widens the bronchial tubes). If your lung function improves after use of a bronchodilator, it's likely you have asthma.¹

Additional tests may be needed to help diagnose asthma, including:

Allergy tests – skin or blood tests to check if you’re allergic to pets, dust, mould, or pollen
Imaging tests – e.g., X-rays to identify any structural abnormalities or infections
Saliva tests – looking for certain white blood cells in the mixture of saliva and mucus released during coughing
Exercise/cold tests – measures your airway obstruction before and after vigorous exercise or taking breaths of cold air
Asthma triggers – if they slightly narrow your airways, you likely have asthma¹

Your doctor may also perform a physical exam to rule out other possible conditions, such as a respiratory infection or chronic obstructive pulmonary disease (COPD).¹

References

Mayo Clinic. Asthma. Available at: https://www.mayoclinic.org/diseases-conditions/asthma/diagnosis-treatment/drc-20369660. Last accessed April 2025.
NHS. Fractional Exhaled Nitric Oxide (FeNO). Available at: https://www.england.nhs.uk/aac/what-we-do/innovation-for-healthcare-inequalities-programme/rapid-uptake-products/fractional-exhaled-nitric-oxide/. Last accessed April 2025.