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How is Asthma Diagnosed?

Updated on February 25, 2012

How is Asthma Diagnosed?

Asthma is diagnosed in three ways. First, from the story given by the parents of recurrent cough, wheeze or multiple episodes of so-called bronchitis. Secondly by observations made on examination of the child and thirdly by a number of tests.

Most children will need very few tests done to make the diagnosis of asthma.

It is important that parents and the child give the doctor as accurate an history as possible. Attention should be paid to times at which wheezing or coughing occurs, factors which may provoke these episodes, how long they last and if they are related to exercise. Your doctor will also ask if other members of the family have asthma, hay fever or eczema. These three conditions all have an 'allergic' or 'atopic' basis and as bronchial hyper-reactivity is partly inherited, this information will help in making the diagnosis.


Many children, when they appear before the doctor will be quite well and nothing will be found on examination. In an acute attack, they will be wheezing and in chronic severe cases there may be a barrel chest deformity.

This means that the chest is more circular ('barrel like') than usual and is due to a chronic trapping of air in the lungs because of the difficulty in breathing out- the hallmark of asthma.


The basic test to diagnose asthma is called a 'lung (pulmonary) function' test. There are a number of ways in which this can be done. As the main problem in asthma is breathing out (expiration), the patient is asked to blow into a machine to see how well he or she can breathe out (exhale). The simplest version of this test is the 'peak flow' test. Young children, less than six to seven years of age, may not be able to cooperate sufficiently to do these tests.

Pulmonary Function Tests

As mentioned above, the child would be asked to blow into a peak flow meter or perhaps a more sophisticated device. This is like blowing into a balloon. The child's performance over three or four blows would be compared to known normal values for his or her age and size. If peak flow is diminished this supports the diagnosis of asthma. It should be remembered that asthma comes and goes and when the child has the test done he or she may be quite well and the test normal. A normal peak flow at one particular time does not exclude asthma. Your doctor may ask you to monitor your child's breathing at home with such a device.

Response to Bronchodilators

To go one step further, in a patient in whom a low peak flow has been demonstrated, a bronchodilator (a medication which dilates and widens the bronchi) can be given and the test repeated.

In general, a medication such as salbutamol (Ventolin) or terbutaline (Bricanyl) would be given by nebuliser and the test repeated in ten minutes. An improvement in peak flow of 15-20 per cent means that the bronchi have responded to the medication and this also strongly supports the diagnosis of asthma.

Exercise Provocation

Another way of making the diagnosis, especially in children with exercise induced asthma, is to do a peak flow test before and after exercise in order to demonstrate a fall in peak flow. The next step is to give some medication before the exercise and to show that it stops, or at least decreases, the previously demonstrated fall in peak flow. In some patients, it may be necessary to give either histamine or metacholine which will produce a fall in peak flow in asthma.

Skin Tests

These are to define the cause of the patient's allergy. A small amount of allergen extract (pollen, cat fur etc.) is placed on the skin which is then pricked gently. If the patient is allergic to a particular allergen, a small itchy swelling will appear. These tests are not entirely reliable and do not always match up with what the patient or the parents identify as the allergen. The other problem is that in some patients there are multiple allergies about which nothing can be done. Skin tests are uncommonly done in children and should be restricted to ery specific situations.

RAST tests

A RAST test (short for "radioallergosorbent test") is a blood test to identify allergens. It is a relatively new test and therefore quite fashionable if rather expensive. Its role in the management of childhood asthma is extremely limited and has no benefit over skin tests.

Chest X-ray

The chest x-ray can help in the diagnosis of asthma, but should only be done when necessary. There is a tendency to do a chest x-ray every time an asthmatic gets sick or is admitted to hospital, which is quite unnecessary. The chest x-ray may show evidence of air trapping, as the patient has trouble in breathing out and because of the thick mucus produced in some patients with asthma, there may be some bronchi which are blocked. The lung beyond may also become deflated (airless) and then collapse (atelectasis). This is quite common in asthma and can be mistaken for areas of infection. In general, atelectasis will resolve with breathing exercises and physiotherapy; antibiotics are not needed.

Trial of Medication

In some patients, especially those with recurrent cough and no wheeze, it may be wise to give some bronchodilator medication for a month or so. If the symptoms disappear, then the diagnosis is asthma.

In summary, asthma can usually be diagnosed quite simply with a very limited number of tests. Remember that a recurrent cough may, in some children, be the only symptom of asthma.


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