Allergy skin tests as a method of diagnosing allergy in children

Copyright (c) 2006 Radoslaw Pilarski

Similarly like in the case of adults, skin tests can be performed in children of any age, but we know from experience that the skin of a child up to the 3rd year is very sensitive and reacts strongly to allergens, often in such a way that makes it difficult to make correct diagnosis.

One of the most common allergen upsetting small children and infants is the food (ingested allergene). It affects the child’s skin more seriously than inhaled allergens. It may lead to an incorrect readings of tests performed in small children. The skin tests carried out on older children are much more reliable. A positive skin reaction occurs if a person is allergic to a particular allergen and if the mast cells of the skin contain the appropriate IgE antibody to this substance. During the skin tests, the mast cells of the skin undergo a process of degranulation in which histamine is released. It is the histamine which causes skin changes such as erythema or bumps. The mast cells of a child’s skin contain smaller amount of antibodies and this may be the reason why we cannot observe a stronger reaction to the allergen.

Skin tests performed in an allergic child may turned out to be negative, when the child is administered antihistamines. Different medicines can also slightly influence the results of the tests. That is why it is necessary to know what medicines has been administered to the child by the child before scheduling the skin tests.

The following medicines should not be taken by the patient during the period preceding the skin tests:

* first-generation antihistamines (Clemastine, Fenergan, Diphergan, Hydroxyzinum) – for 3-4 days
* cetirizine (Zyrtec) - for 3-4 weeks
* loratadin (Claritine) - for 2-3 weeks
* astemizol (Hismanal) – for 4-6.weeks
* ketotifen (Zaditen, Pozitan) – for 2 weeks,

There are three main types of skin tests:

* puncture, scratch test ("prick test", percutaneous).
* intradermal test (intracutaneous),
* patch test (epicutaneous)

Patch tests are performed by a dermatologists if a contact allergy is suspected or other reactions typical to occupational allergy. The results can be obtained in 48 or 72 hours. Children are diagnosed using scratch tests. Such tests are considerably easy to perform and correlate well to the specific IgE antibodies in serum and to the appropriate inhaled provocative test under the influence of the same allergen. Skin reaction can be observed in 5-10 minutes after placing the allergen onto the surface of the skin.The test is carried out by placing a small amount of the suspected allergy-provoking substance on the skin. Next, the skin is gently scratched through a small drop of the allergen. The scratched place should not bleed. The results of the test can be collected in 15-20 minutes or even earlier if the blisters extend rapidly and form protrusions, which indicates an occurrence of pseudopodia.

The best place to perform a skin test is the inner part of the forearm or one’s back. Children should be diagnosed by performing the tests on their forearm unless there are no visible skin changes. The skin tests can be considered to be done correctly if a positive control test (1 per cent solution of histamine) and a negative control test are carried out. The last one uses saline extra phenol added to the suspension of allergens as a preservative. If the results of the positive control are negative, the tests cannot be regarded reliable. In such a case, the reaction of the skin is inhibited by antihistamine medicines.

Such tests should be repeated after some time, having carefully examined the methods of the child’s treatment within the last period of time. Sometimes, the reaction with the solution of sodium chloride can turn out to be positive. It may happen if the reagent has been contaminated or the child displays some symptoms of dermographism.

The extent of the reaction is determined by measuring the diameter of the blister using a special transparent scale. This is the most accurate testing method. A control histamine testing will produce a 5-7mm blister. In order to confirm a positive result of the test, the diameter of the allergene-provoked blister should not be smaller than 3mm. All tests carried out without positive and negative control testing are unreliable. Another method of determining the results of tests is marking the examined places with (+) or (-). If the diameter of the blister is at least 5mm, the histamine test result is marked with a triple pluses +++. In this method, which is not recommended, the smallest positive place is labelled with ++, which corresponds to a blister allergic reaction of 3mm. The blisters usually disappear after several minutes or after an hour. In some cases, after 4-6 hours, a delayed reaction may occur.

During intradermal tests, a small amount (0,05-0,1 ml) of a special allergen solution is injected into the skin. The readings should be made after 15-20 minutes. This test is more painful and it produces more false positive results.

Skin tests, which are the basic methods of diagnosing allergies, cannot be carried out on patients suffering from severe allergic reactions (e.g. an allergic shock resulted from a contact with allergen mentioned in the case history). In such situations, a safe method of diagnosis consists in isolating characteristic IgE in the blood. In some very rare cases, during performing the skin tests an anaphylactic reaction to an allergen may occur. Although such reactions are usually harmless, it is necessary for an allergist to have an anti-shock medical kit at hand in order to help the patient if necessary.