Diagnosis and Testing
What is Allergy?
The term allergy is used to describe a particular response by the body’s immune system to a substance in the environment, such as pollen or foods. In most people, the body recognises these substances as harmless. However, some people have a tendency to develop allergy (this is called atopy). In these people, exposure to these substances (called allergens) results in the production of IgE antibodies against the allergens. This process is called sensitization. However, at this stage, no symptoms are noted.
The next time the person meets this substance the antibody reaction will cause the release of certain chemicals into the body. These chemicals cause the symptoms of allergy.
What is the Difference Between Allergy, Intolerance, and Sensitivity?
Although the word allergy is commonly used to describe any unpleasant reaction to a drug, food, insect sting or chemical, this can be misleading. The word should only be used to describe a reaction produced by the immune system to a normally harmless substance.
Sensitivity is an exaggeration of a normal side effect produced by contact with a substance. For example, the amount of caffeine in a single cup of coffee may cause palpitations and trembling in a sensitive person, where this would normally only occur after far higher doses of caffeine.
Intolerance happens for a variety of reasons, for example, because your body does not produce sufficient quantities of a particular enzyme/chemical, which is needed to break down a food and aid digestion. However, intolerances do not involve the body’s immune system and cannot, therefore, result in life-threatening allergic reactions.
How Do You Know If You Have an Allergy?
The most useful tool in deciding whether someone is allergic is to take an ‘allergy history’. A good allergy clinician can usually identify the likely allergens from the history alone, and allergy tests may not be needed. However, there are occasions when tests can be useful to confirm the diagnosis. This is especially important if you have had a severe reaction and if there is any confusion as to whether your symptoms are caused by a true allergy or whether some other process is involved.
What Allergy Tests are Available?
The type of test to be carried out will depend upon your symptoms, the condition of your skin, and any medication you are taking.
The best allergy test (known as a ‘gold standard’ test) is to expose a person to the allergen concerned, such as a food. This is known as an Allergy Challenge test. Since this may cause an allergic reaction, such a test should generally only take place in a specialist clinic where appropriate medical facilities are available to manage any reaction.
Unfortunately, the allergy challenge test can be complicated to organise and may not be safe for you. Therefore, other tests which look for the presence of IgE antibodies are often used. These tests look for sensitisation rather than true allergy, as some people will be sensitised to the allergen but not have developed the allergy (see above). This is because in these people, the degree of sensitisation which has occurred is not sufficient to trigger an allergic reaction. Indeed, in some cases, such a person may become allergic if they stop eating a food to which they are sensitised.
It is for this reason that allergy tests need to be interpreted by a healthcare professional qualified in allergy, who will interpret the results in light of the patient’s history in order to determine whether someone is allergic. This also explains why it is important not to test everybody for every known allergen, as it can be difficult to interpret the results.
Patch Testing can be useful in cases of contact dermatitis (eczema) where delayed–type allergy is suspected to be a trigger. They are not helpful in the diagnosis of immediate-type allergies, including most food allergies.
Other (non-conventional) allergy tests are not considered to be relevant, standardised or repeatable and are considered to have no place in the diagnosis of allergy. These include Applied Kinesiology (measures muscle strength), Auricular Cardiac Reflex Method (measures strongest pulse at wrist), Hair Analysis, Leukocytotoxic Tests and Vega Testing (measures the electromagnetic fields produced by the sufferer).
IgG blood tests are one type of non-conventional test. These are different from blood tests for specific IgE, and are of no use in the diagnosis of true allergy.
Can We Test For Food Intolerance?
Some practitioners use IgG blood tests in helping to diagnose food intolerances, such as those which may cause symptoms in some cases of migraine, arthritis and irritable bowel syndrome. However, the data is not scientifically robust and most allergy specialists consider IgG blood tests to be unhelpful in these circumstances. The ‘gold standard’ test for food intolerance remains the elimination and challenge diet (see Food Intolerance section).
Video on the Subject
Last updated: March 2012