|
|
|
Allergy Skin Tests As A Method Of Diagnosing Allergy In Children
by: 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 alergens, often in such a
way that makes it difficult to make correct diagnosis.
One of the most common allergen upsetting small childern and
infants is the food (ingested allergen). 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
allergi. 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. Childern 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
labeled 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.
Copyright (c) 2006 Radoslaw Pilarski
|
Informative Articles :
|
|
|
|
