Diagnosing asthma

If you have typical asthma symptoms, your GP is likely to be able to make a diagnosis. Your GP will want to know when your symptoms happen and how often, and if you have noticed anything that might trigger them.

A number of tests can be carried out to confirm the diagnosis.


A breathing test called spirometry is carried out to assess how well your lungs work. You will be asked to breathe into a machine called a spirometer.

The spirometer takes two measurements: the volume of air you can breathe out in one second (called the forced expiratory volume in one second or FEV1) and the total amount of air you breathe out (called the forced vital capacity or FVC).

You may be asked to breathe out a few times to get a consistent reading.

The readings are compared with average measurements for people your age, which can show if your airways are obstructed.

Sometimes an initial set of measurements is taken, then you are given a medicine to open up your airways (a reliever inhaler) to see if this improves your breathing. Another reading is then taken and, if it is much higher after taking the medicine, it can support the diagnosis.

Peak expiratory flow rate test

A small hand-held device known as a peak flow meter can be used to measure how fast you can blow air out of your lungs in one breath. This is your peak expiratory flow rate (PEFR), and the test is usually called a peak flow test.

You may be given a peak flow meter to take home and a diary to record measurements of your peak flow. Your diary may also have a space to record your symptoms. This will help you recognise when your asthma is getting worse.

Other tests

Some people, but not all, may need more tests. The tests may confirm the diagnosis of asthma or help diagnose a different condition. This will help you and your doctor plan your treatment.

Airway responsiveness tests

This test is used to see how your airways react when they come into contact with a trigger. You will be asked to take a mannitol challenge test which involves breathing in increasing amounts of a dry powder. This deliberately triggers asthma symptoms and causes the airways to narrow. In children, exercise is sometimes used as a trigger.

You then blow into the spriometer to measure how much your FEV1 and FVC have changed in response to breathing in the trigger. If there is a significant decrease in these measurements, you may have asthma.

Testing airway inflammation

  • Phlegm sample. The doctor may take a sample of phlegm to check whether you have inflammation in the lungs.
  • Nitric oxide concentration. As you breathe out, the level of nitric oxide in your breath is measured. A high level of nitric oxide can be a sign of airway inflammation.

Allergy tests

Skin testing or a blood test can be helpful to confirm whether your asthma is associated with specific allergies, for example dust mites, pollen, or foods.

Read more information about diagnosing an allergy.

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Occupational asthma

If you report that your symptoms are better on days you do not work or when you are on leave, you may have occupational asthma. Occupational asthma may also be diagnosed if you work in an industry where there is a high risk of getting the condition, such as:

  • paint sprayers 
  • bakers and pastry makers 
  • nurses 
  • chemical workers 
  • animal handlers 
  • welders
  • food processing workers 
  • timber workers

To help diagnose occupational asthma, your GP may ask you to take measurements of your peak expiratory flow both at work and when you are away from work.

Your GP may then refer you to a specialist in occupational medicine to confirm the diagnosis.

Tests can also be carried out to see if you are allergic or sensitive to certain substances known to cause occupational asthma.

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