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Skin Tests for Allergy
The commonest type of skin test, most often used to
identify the causes of inhaled allergy and immediate food allergy, is the skin
prick test. When contact allergy is suspected the usual
test is the patch test.
Certain kinds
of allergic reaction; for example, allergy to certain drugs, and allergy to moulds, yeasts and
fungi; may only show up using an intradermal
test an alternative to the skin prick test that is less commonly used in Europe
than in the USA.
Skin Prick Test
This test involves placing separate drops of solutions of allergen
extracts onto the skin of your forearm or back and then using a very fine lancet to prick
through the drop into the skin. A positive test results in a small raised wheal with a red
surrounding flare. Results are known within fifteen minutes and the
test is painless. It may be necessary for you to stop certain
medications such as anti-histamine medications before a skin prick test
but you should only do this under the supervision of a doctor. Oral
or inhaled steroids do not interfere with skin prick tests.
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An allergy skin prick
test
A small drop of a solution
containing a dilution of the substance under test is placed on the forearm, and a lancet
is used to gently prick the skin through the solution.
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A positive allergy skin prick test
If the patient is allergic
to the allergen being tested, the skin on the arm will swell with fluid (the paler
area), and get itchy and red in the surrounding area. The reference tests are on
the left: Histamine (positive control) and the diluent (negative control).
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This method of testing is excellent for certain conditions
and certain allergens, although it is less exact for others. Careful comparison between the
results of skin tests and the results of challenge tests, either by inhalation (bronchial
challenge) or by ingestion (food challenge) have shown that the skin test may in some cases
over-diagnose (false positive) or in some cases under-diagnose (false negative). For
this reason it is important that diagnosis based on the skin prick test should consider the
result alongside the patients own experience (the 'clinical history').
It is also important to understand that food intolerance
(delayed or 'hidden' food allergy) does not show up using this method, and
will usually only show up by the use of an elimination
diet and food challenge test. Such 'false negative' results to skin prick tests probably occur
because the reaction to the food being tested is either not an allergy at all (for example a toxic
effect or an enzyme deficiency as in lactose intolerance) or it is an allergy, but one that works in a way that, as yet, we do not
fully understand.
Skin Prick Testing for Drug Allergy: Currently,
only limited tests are available to diagnose specific medication allergy. Allergy skin
testing to determine the presence of IgE antibody is available for penicillin and insulin.
Skin testing is sometimes done for other medication, but is is often of limited accuracy.
Intradermal Skin Tests
 This
is more sensitive than the skin prick test in certain situations. 0.05 — 0.1ml of
the substance is injected into the very outermost layer of the skin of the arm. The result
can be read at 20-30 minutes and in some cases at 24 hours and at 48 hours.
A positive reaction at 30 min is suggestive of immediate type (type I) allergy and
positive reaction at 48 h indicates delayed-type hypersensitivity (type IV).
This test is done much less frequently than the skin prick test (although
it tend to be more popular in the USA), because of a concern that it may give false positive results. Also the risk of general
anaphylaxis, although still very low, is a little higher than in prick testing because of the
larger amount of allergen injected. However, once better validated, it may be a way to diagnose certain kinds
of allergic reaction; for example, allergy to certain drugs, and allergy to moulds, yeasts and
fungi.
Patch Tests
Patch testing can be used to assist in the diagnosis
of contact dermatitis and may also be useful to assist in the diagnosis of certain types of drug allergy. In a recent development, patch testing has also been
used to try to assist in the diagnosis of delayed food allergy; a condition where the standard skin prick test may give a falsely negative
reading. However, this form of testing remains to be fully evaluated and is only a
research technique at present.
Contact eczema (contact dermatitis) is
caused by an allergic reaction to something with which the patient is in regular
contact. A very wide range of substances is no known to be capable of inducing such
reactions. A profile of common allergens
is given below. From the nature of these it can be seen that particular occupations
pre-dispose the workers to contact sensitivity problems.
 The diagnosis
of contact dermatitis (for example, a skin allergy to a cosmetic, a metal such as nickel, or a
chemical in rubber) requires a 'patch test'. This is because the usual allergy antibody
(IgE) is not involved, and a skin prick test or allergy blood test would not pick it up.
The reaction is thought to be related to a response of the white blood cell rather than an
antibody response, and the reaction therefore take a day or two 
to develop. Several substances to which the patient may turn out to be sensitive are
placed into tiny aluminium chambers mixed into a gel and these are then attached to the skin
for 48 to 72 hours to determine whether an area of itchy rash develops, the sign of contact
allergy. Patch tests responses need to be interpreted with care to distinguish which
show an allergic and which show an irritant reaction.
To find out the cause of contact
eczema in the individual patient can require close and careful detective work. First the
doctor needs to take a very careful history including inormation about the patient's work and
hobbies. A range of standard contact 'patch' tests
is then done supplemented, if necessary, by additional tests with items to which the patient
becomes exposed on a regular basis. For example, a hairdresser with contact eczema would
have patch tests to the various hair treatment chemicals used in her job. The following
list includes some of the most common skin sensitisers and shows where they are found.
Drug Allergy: Patch Testing can be
useful in some patients who have a delayed reaction to drugs. Drugs that can be tested by
patch test include:
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Drugs that can be tested by patch test
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Ampicillin
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Paracetamol
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Clindamycin
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Macrolide Antibiotics
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Acyclovir (Zovirax)
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Hydrocortisone
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Terbinafine
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Neomycin
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Profile of Common Allergens for Contact Allergy Testing
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Allergy Clinic
