Diagnosis of Allergy

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The diagnosis of the allergy is made with the data given in detail by the patient, which is complemented by the objective signs of the physical examination.

In accordance with the characteristics of the clinical picture, a suspicion is established about the immune mechanism involved and the phase in which the allergic disease is found. Depending on the suspected mechanism, some tests or others are performed to arrive at an etiological or cause diagnosis.

To identify which substances cause the allergy, a series of in vivo tests are available such as skin tests –prick test or intraepidermal test-, intradermal tests, epicutaneous or patch tests; and, controlled exposure tests with the suspected causative agent. There are also in vitro tests such as blood tests in which the presence of specific IgE or the basophil activation test is evaluated.

The molecular diagnosis using skin tests (prick test or intra-epidermal test) or determination of specific IgE in blood can in some cases identify the protein responsible for IgE mediated reactions with which it can improve the sensitivity of the diagnosis in some cases and the stratification of risk (possibility of severe reactions or not in the case of a new exposure, amount of allergen that is capable of inducing reactions,…).

Currently, there are tests (micro-matrix platforms) that allow you to study more than 100 allergens (proteins) that belong to more than 50 different substances (allergenic sources) that include food, mites, pollens, fungi, epithelia, parasites, and Hymenoptera venoms, using a very small volume of blood.

Tests for diagnosing Allergy

Folder with a patient's medical history

Clinical history.

Allergy test on the arm

Intraepidermal test (prick test). It is used to investigate immediate or IgE mediated hypersensitivity. Food allergens, Hymenoptera poisons, inhaled allergens such as dust mites, tree and plant pollen, fungi, animal epithelia, and any other allergen can be studied. It consists of applying a drop of the extract of the allergen to be evaluated on the forearm and the realization of a small puncture with a lancet. In the case that the patient is sensitized (positive test) a small wheal appears (inflammation, redness, and itching) in the puncture area. They are very safe, sensitive and fast tests (20-30 minutes).

Needle pricking a forearm in an intradermal test

Intradermal test. It is used to investigate immediate and also delayed reactions. It is usually used to study medications. It involves injecting a small amount of the allergen into the dermis (similar to a tuberculin test).

Person with patches on the back by skin test

Epicutaneous test (patch test). It used to investigate delayed hypersensitivity. It consists of applying on the skin, one or several of the allergens responsible for contact skin reactions. The patch is left on for 48 hours. It can also be performed with drugs.

Blood collection tube

Blood tests for the study of tryptase (useful during anaphylaxis) and specific IgE (against inhaled allergens and food).

More viscous mucus or mucus in the nose

Nasal, conjunctival or bronchial exposure test with an allergen. It is used when we suspect immediate reactions. It consists of applying the allergen inside the nose, eye or bronchus (lung) respectively. It is used to establish what symptoms the allergen produces when breathing or coming into contact with the eyes. It is usually done to confirm which allergen is responsible for the symptoms in a patient who has several positive tests.

Nut allergy

Oral exposure with foods tests. The patient has to take, in a controlled way, a portion of food that we suspect could have caused an allergic reaction, for confirmation or discard. Small amounts of food are given, which gradually increase until reaching a normal ration. It is considered a risk test, so it should always be carried out in a hospital with trained personnel to counteract any possible reaction to the food to study. Small amounts of it are given and at each specific time, the quantity is increased until reaching the normal dose of the food.

Green, blue and white striped tablets

Test of oral, subcutaneous, intravenous, or intramuscular exposure to drugs. It consists of the patient taking, in a controlled way, a medication that is suspected of causing an allergic reaction. The medication can be taken orally, subcutaneously, intramuscularly or intravenously. Risk tests are considered, so they should always be performed in a hospital with trained personnel to counteract any possible reaction.

Substantiated information by:

Joan Bartra Tomàs
Rosa Maria Muñoz Cano

Published: 24 July 2019
Updated: 24 July 2019

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