For people who suspect that they are suffering from an allergy of some form, a consultation with an allergist is essential to identify the guilty allergen (or allergens) and confirm its causal role in the occurrence of the symptoms.

Once the guilty allergen has been identified, desensitisation (or allergen immunotherapy) can be embarked upon, as it is the only treatment that can modify the natural course of allergic disease.

The different steps in the diagnosis of allergy:

• The interview
• Skin testing
• Blood tests 
• Provocation tests
  
  
  
  
  

The interview (clinical history or anamnesis)

A rigorous, detailed interview is the keystone of the first consultation. The aim is to confirm the existence of an allergic state and establish a preliminary list of allergens which might be causing the observed symptoms.

The main areas covered are:

• History of the symptoms (e.g. how long ago they first appeared, when they appear, any triggering factors already identified, etc.);

• Family history of allergy (e.g. any close relative who is allergic) or in the patient own history (e.g. allergy during childhood)
• The patient's general environment (e.g. living and working conditions, climate, etc.)
• Details about lifestyle and habitats (home, workplace, pets, eating habits, etc.)
• Activities (occupation, pastimes, etc.)

More than one consultation may be required to gather all of the information required. This process might seem tedious, but this assessment is essential to the diagnosis of the allergen involved.

Physical examination

As well as a general examination (weight, blood pressure, etc.), the specialist examines in detail those organs in which allergic symptoms commonly manifest, notably the skin, lungs, nose and bronchi.

This allows the allergist to assess the impact of the allergy.

It indicates possible further tests that might fall outside the allergic field (pulmonary function tests in asthma).

Skin testings

After the interview and physical examination, a list of allergens that might be responsible is compiled in order to narrow down the field for skin testing.

What does skin testing involve?

Download the Flash animation: the different steps for skin testing ("skin prick test")

The point of a skin test is to try to reproduce on a very small scale on the patient's skin the allergic reaction in its "immediate hypersensitivity"form (the allergic reaction appears immediately after contact with the allergen involved).

To do this, a drop of the suspected allergen is placed on the skin and then, using a special lancet, a tiny puncture is made in the skin at the center of the drop.

 
If a mosquito bite-like reaction develops where the allergen was deposited, it means that the patient's body has reacted, and that the allergen being tested might be responsible for symptoms. The intensity of the allergen-induced reaction is gauged by comparison with the reaction induced by some "control" substance which induces a reaction in all subjects without exception. Such a control is necessary to give a measure of the patient's baseline reactivity.

Skin testing:

• Fast and painless, it is the technique most commonly used
• A drop of allergen is placed on the skin of the forearm or on the back. Then a "micro" prick is made in the center of the drop so that the allergen can penetrate into the dermis and trigger the allergic reaction.
• The results can be read after 20 minutes: a hypersensitivity reaction results in redness of the skin resembling a mosquito bite with edema (swelling) and itching.
• The severity of the reaction is evaluated compared with the reaction caused by a "control" substance (a "control" substance causes a reaction in all individuals without exception and makes it possible to learn the level of severity with which the body of the patient reacts).

This technique is fast, painless, reproducible over time and well-tolerated, even by very young children.

Blood tests

If the results of skin tests are inconsistent with the clinical picture, or if carrying such tests out presents a problem (e.g. because of extensive eczema or because the patient is already taking drugs like antihistamines that tend to inhibit this type of reaction), the physician may order blood tests. The most common test is quantitative determination of the levels of specific IgE (the class of antibody which is involved in allergic reactions) directed against the suspected allergen or allergens. This assay corresponds to a direct measurement of the specific pathological basis of the problem and yields a clear response as to which allergen is causing the symptoms.

Provocation tests

As their name suggests, the purpose of these tests is to elicit an allergic reaction by directly bringing the allergen into contact with the "target"organ, which is the site of allergic symptoms:

• Inhalation of the allergen in case of a respiratory allergy, or asthma
• Spraying into the nose in case of rhinitis;
• Application on the conjunctiva ("whites" of the eye) in case of conjunctivitis
• Ingestion in case of a food allergy

Not performed systematically, these tests are carried out only in certain cases (for example, if doubts remain after all the other previously-mentioned tests have been performed), and, it goes without saying, under close medical monitoring.

Patch tests

The principle of a patch test is based on reproducing the allergic reaction in its "delayed hypersensitivityform" on the patient's skin (the allergic reaction appears several days after contact with the antigen involved).

To do so, patches containing small amounts of the suspected antigen are applied to the skin of the back and left in place for 2 days.

An analysis grid makes it possible to interpret the results of the test by identifying each of the antigens. The different types of reaction can range from a negative reaction to an extremely positive reaction. A positive reaction means that the body reacts when confronted with the antigen tested and that the antigen may be responsible for contact dermatitis.

Patch test:

• Easy to use, it is a reliable technique that makes it possible to obtain reproducible results.

• The clinical signs of a positive reaction are erythema (redness), oedema (swelling), papules (small pimples on the skin), and vesicles (small pimples on the skin filled with fluid), which may appear at the application site of the patches.
• Applying the patch test is fast and painless.

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