Asthma - tests that you may have to undergo

Written by The National Asthma Campaign, and Dr Martyn Partridge, consultant physician



On first diagnosis of asthma, or indeed to aid with diagnosis, there are a number of tests that you may need to have to provide a more detailed picture of your condition. You may be asked to take some lung function tests which will give a more precise view of what is going on in your lungs and airways, as there may be a number of reasons why you are experiencing breathlessness.

Spirometry

Spirometry is a simple test that can be carried out at your doctor's surgery or in an asthma clinic. It is useful in the initial evaluation of people experiencing breathlessness, who may have one of a variety of respiratory or cardiac disorders.

A device called a spirometer measures the actual volume of air breathed out in the first second of exhalation and gives a reading known as FVC (forced vital capacity). This is then given in terms of a percentage, FVC ratio, which can be compared against charts that give the normal reading for a person's height and age. In most 'normal' subjects, more than 75 per cent of the FVC will have been exhaled within one second, although this 'normal' figure may decline with age.

Spirometry detects one of two types of lung disease:

obstructive, which is connected with the airways - asthma is one such type

restrictive - diseases of the lung that lead to the lungs becoming small, such as:

sarcoidosis or fibrosing alveolitis.

disorders of the chest wall such as scoliosis (curvature of the spine) which restricts the size and expansion of the lung.

obesity which similarly leads to small lungs.

Both obstructive and restrictive disorders may show the symptom of breathlessness, but wheezing is only likely with obstructive disorders.

Peak flow

Current British Guidelines for the assessment and management of asthma are based on peak expiratory flow (PEF). In simple terms a peak flow measurement shows how hard and quickly you blow air out of your lungs. The meter has a marker that slides up the scale as you blow out. The better controlled your asthma, the harder you will be able to blow out and the higher your peak flow score will be. Peak flow readings are measured in litres per minute.

Measurement of peak flow is valuable for objectively measuring how severe your asthma is, and is routinely used once asthma has been diagnosed to monitor the condition. It is not as effective as a test to diagnose asthma because peak flow can be moderately reduced at times for reasons other than asthma.

Sometimes peak flow can appear almost normal even where there is severe lung damage present, such as in emphysema. This reflects what part of the lung is affected by the disease. In emphysema the peak flow is well maintained but because the air sacs in the lung are severely damaged, air gets trapped beyond the airways in the periphery of the lung, which collapses on breathing out because of loss of lung elasticity.

Chest X-rays

Chest X-rays are sometimes used to aid the initial diagnosis and help to differentiate asthma from other lung diseases.

Allergy tests

Allergy is increasingly recognised as a cause of asthma and an important part of asthma management is the identification of a person's 'triggers' and the avoidance of those triggers.

Sometimes a doctor may recommend that you undergo allergy tests to help identify exactly what your triggers are and the degree to which they affect you. This will then help you to try and manage your asthma through the avoidance of the allergen, where practicable.

This knowledge can also help your doctor to tailor your asthma treatment and advise you of where and when you are likely to experience worsening asthma symptoms and what action you can take to reduce the severity. The most common tests used to diagnose allergy are skin prick tests.

Skin prick tests

Skin prick tests are a simple and effective way of assessing allergic triggers. They are cheap to do, give quick results, and are extremely safe.

However, it is worth remembering that positive skin prick tests have to be interpreted alongside the clinical story and are often only significant if there is a history of wheezing on exposure to allergens.

Small samples of the suspected allergen are tested on the skin, usually on the forearm. Additionally a control test is used so that any reaction to allergens can be clearly measured. Two types of control test are used.

One just contains the solution in which the allergens are dissolved because some people have sensitive skin and can react to the pressure of the prick alone.

The second is histamine to ensure that a reaction will occur if a response to an allergen leads to a histamine being released. The test is painless and results are usually available within 15 minutes of having the test. A person who is sensitised to the allergen will display a reaction on the skin where the test was performed. This skin reaction is seen as redness, swelling and itch. A positive test will produce a skin reaction that measures 3mm more in diameter than the area where the control test was done.

Although these tests are generally accurate, there are some medications that can affect the results. If you are taking antihistamines you will normally be asked to stop taking these at least 48 hours before the test. Some other medications such as antidepressants can also interfere with skin prick tests. For these reasons it is sometimes also necessary to have a blood test.

Blood tests

Blood tests cannot help with the diagnosis of asthma but like skin prick tests can be used to look for specific allergies. They require a sample of blood to be taken so that it can be sent to a laboratory for testing. Blood tests are slightly less sensitive than skin prick tests and results usually take a few days to a week.

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