If you have typical asthma symptoms, your GP will often be able to make a diagnosis.

They will want to know when the symptoms happen and how often, and if you have noticed anything that might trigger them.

Your GP may also ask whether you have any allergic (atopic) conditions such as eczema and hay fever, which often occur alongside asthma.

A number of tests can be carried out to confirm the diagnosis. These are described below.

In children, asthma can be difficult to diagnose because many other conditions can cause similar symptoms in infants. The tests outlined below are also not always suitable for children.

Your GP therefore may sometimes suggest that your child uses an asthma inhaler as a trial treatment. If this helps improve your child's symptoms, it is likely they have asthma.

Spirometry

A breathing test called spirometry will often be carried out to assess how well your lungs work. This involves taking a deep breath and exhaling as fast as you can through a mouthpiece attached to a machine called a spirometer.

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

You may be asked to breathe into the spirometer a few times to get an accurate reading.

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

Sometimes an initial set of measurements is taken, and you are then given a medicine to open up your airways (a reliever inhaler) to see if this improves your breathing when another reading is taken.

This is known as reversibility testing, and it can be useful in distinguishing asthma from other lung conditions, such as chronic obstructive pulmonary disease (COPD).

Peak expiratory flow 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 (PEF) and the test is usually called a peak flow test.

This test requires a bit of practise to get it right, so your GP or nurse will show you how to do it and may suggest you take the best of two or three readings.

You may be given a peak flow meter to take home and a diary to record measurements of your peak flow over a period of weeks. This is because asthma is variable and your lung function may change throughout the day.

Your diary may also have a space to record your symptoms. This helps to diagnose asthma and, once diagnosed, will help you recognise when your asthma is getting worse and aid decisions about what action to take.

To help diagnose asthma that may be related to your work (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 to confirm the diagnosis.

Other tests

Some people may also need a number of more specialised tests. The tests may confirm the diagnosis of asthma or help diagnose a different condition. This will help you and your doctor to plan your treatment.

Airways responsiveness

This test is sometimes used to diagnose asthma when the diagnosis is not clear from the more simple tests discussed above. It measures how your airways react when they come into contact with a trigger.

You will be asked to breathe in a medication that deliberately irritates or constricts your airways slightly if you have asthma, causing a small decrease in your FEV1 measured using spirometry and possibly triggering mild asthma symptoms. If you do not have asthma, your airways will not respond to this stimulus.

The test often involves inhaling progressively increasing amounts of the medication at intervals, with spirometry measurement of FEV1 in between to see if it falls below a certain threshold. In some cases, however, exercise may be used as a trigger.

Testing airway inflammation

It may also be useful in some cases to carry out tests to check for inflammation in your airways. This can be done in two main ways:

• a mucus sample – the doctor may take a sample of mucus (phlegm) so it can be tested for signs of inflammation in the airways
• nitric oxide concentration – as you breathe out, the level of nitric oxide in your breath is measured using a special machine; a high level of nitric oxide can be a sign of airway inflammation

Allergy tests

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

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

Read more about diagnosing allergies.

Want to know more?

• British Lung Foundation: diagnosing asthma