Asthma - Diagnosis

There's no single test for asthma, but it can usually be diagnosed from your symptoms and some simple breathing tests.

Your GP will often be able to diagnose asthma. But they may refer you to a specialist if they're not sure what's causing your symptoms.

Seeing your GP

Your GP may ask:

• whether you have typical symptoms of asthma, such as wheezing and breathlessness
• when the symptoms happen and how often
• whether you've noticed anything that might trigger your symptoms
• if you have any conditions that often occur alongside asthma, such as eczema or hay fever

The tests below may be used to help confirm the diagnosis, although they aren't always practical – particularly in young children.

If your GP feels tests aren't needed, he or she may just give you or your child an asthma inhaler to use for a short time. If this helps, it's likely you or your child have asthma.

Breathing tests

Two simple breathing tests are sometimes used to help your doctor diagnose asthma.

Spirometry

A test called spirometry can help show how well your lungs and airways are working.

It involves breathing out as fast as you can through a mouthpiece attached to a machine called a spirometer.

The spirometer takes two measurements – the amount of air you breathe out in the first second and the total amount of air you can hold in your lungs.

The readings are compared with a normal measurement for someone of your age, gender and height to work out if your airways are narrow.

Sometimes the test may be repeated a few minutes after taking a puff from an asthma inhaler to see if your result improves. If your breathing tubes are narrow at the time of the test, a better result after using an inhaler means it's likely you have asthma.

Peak flow test

A peak flow test is a simple test to measure how fast you can blow air out of your lungs in one breath.

It involves breathing out as quickly and as hard as you can into a small device called a peak flow meter.

The result is compared to what's normal for someone of your age, height and gender.

You may be given a peak flow meter to take home to record your peak flow over a period of weeks, particularly if asthma is suspected but your peak flow and spirometry are normal when measured, as asthma symptoms and peak flow can vary over time. 

To help diagnose work-related asthma, you may be asked to measure your peak flow at work and away from work.

Other tests

Occasionally, you may be referred to a specialist doctor for further tests to confirm or rule out asthma.

Airway responsiveness

An airway responsiveness test is a test that measures how your airways react to an asthma trigger.

During the test, you'll usually be asked to breathe in a medication that will irritate or narrow your airways slightly if you have asthma. You'll then have a spirometry test to check if your breathing is affected.

In some cases, exercise may be used as a trigger instead of medication.

Testing airway inflammation

Sometimes it's useful to check for inflammation in your airways.

This can be done in two ways:

• a mucus sample – the doctor may take a sample of mucus (phlegm) so it can be tested for signs of inflammation
• 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 inflammation

Allergy tests

If your doctor thinks your symptoms may be triggered by an allergy, they may recommend allergy tests to find out what you're allergic to.

Common allergy tests include:

• skin prick testing – a small sample of the substance you may be allergic to (an allergen) is pricked gently under the skin: if you're allergic, a small blister develops within 15 minutes
• a blood test – a sample of your blood is checked for substances that are produced by your body in response to an allergen

Tests can also be carried out to see if you are allergic or sensitive to substances known to cause work-related asthma.

Want to know more?

• Asthma UK: diagnosing asthma