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:
Clinical history 
The coexistence of symptoms and allergies is verified through an assessment of the allergy's etiology, which is based on a full interview of the patient or his/her family, in order to detect the suspected allergen.
This interview starts with a full summarisation of the patient's medical history and is followed by more precise questions regarding the allergy, as well as the environmental and professional conditions in which it develops.
The presence of a family and/or personal allergic tendency (history of allergic events) is a good indicator of the occurrence of a respiratory allergy. Beyond the necessary history taking, the allergy assessment includes skin tests, lab tests (serum measurement of specific IgE antibodies) and sometimes allergen provocation tests.
Skin testing Skin testing (prick test) conducted by the allergist is the basis for the allergy diagnosis.
The purpose of skin testing is to detect the presence of specific IgE antibodies for an allergen by the "triple response" of Lewis. When the allergen introduced in the dermal layer encounters specific IgE antibodies bound to the surface of mast cells, a reaction occurs activating the cells that release their mediators (histamine, kinine, leukotrienes, etc.), resulting in an immediate reaction characterised by edema, erythema and pruritus. This reaction tops out after 20 minutes. It is sometimes followed by inflammatory cell infiltration (eosinophils) that can last for 4 to 8 hours.
The performance of skin tests has greatly improved, owing to the use of standardised and better-characterised allergen extracts. The skin test technique most commonly accepted is that of the skin prick test. It is reliable, reproducible, relatively inexpensive and non-invasive. A drop of allergen is placed on the skin of the forearm or back, and the skin is pricked through the drop area using a plastic needle.
The results of the test can be read 15 to 20 minutes later, via measurement of the papule diameter of the skin reaction and comparison with that of control solutions (histamine and solvent). A skin test producing a papule with a diameter greater than 3 mm(and > 50% of the positive control) is considered as positive.
Skin prick tests can be performed in individuals 3 months of age or older.
The lab tests The measurement of specific IgE antibodies is indicated as a first resort when skin tests cannot be conducted (extensive dermatosis, dermographism, atopic eczema). They are carried out secondarily in order to confirm the diagnosis of the clinical history, when there is disagreement between this diagnosis and the results of skin tests. In all cases, ordering laboratory examinations should be guided by the clinical context and the skin tests.
Lab tests referred to as “screening” tests
Lab tests referred to as screening tests based on an assay of specific IgE antibodies to a mixture (identified or otherwise) of several allergens have existed for more than 15 years. These tests can serve as a guide for a possible allergic sensitisation, but they have their limits. When the result of the test is positive, it indicates the presence of specific IgE antibodies, but without specification of the allergen. In this case, it is necessary to refer the patient to an allergist for a more precise diagnosis (skin tests or specific IgE single-allergen assays). When the result of the test is negative, it indicates the absence of specific IgE antibodies for the allergens included in the test alone, but does not rule out sensitisation to one or more allergens not appearing in the test. In this case as well, the opinion of an allergist is necessary.
Allergen provocation tests In the event of disagreement between the clinical history, the skin tests and the laboratory examinations, the specialist physician may decide to order or carry out a nasal, ocular or bronchial provocation test with the suspected allergen. These tests have the purpose of artificially stimulating the target organ (nasal mucosa, ocular conjunctiva or the bronchial tubes) using escalating doses of an allergen, in order to reproduce the allergic reaction with its constellation of symptoms. These tests are not systematically performed in routine practice because they are long and demanding for the patient. However, the result of the test, whether positive or negative, makes it possible to confirm or rule out the allergy with certainty. The conditions for conducting tests as well as the allergen administration protocols are currently well-defined.
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