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Written by Dr Victoria Lewis, specialist registrar in dermatology
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Listed below are the most commonly used tests in
dermatology.
RAST test
A RAST (radioallergosorbent) test is a blood test used to
investigate increased sensitivity to a variety of food groups (eg eggs,
cows' milk and nuts), house dust mite and animal dander (hair or fur). It
is often used in the investigation of patients with
atopic eczema.
A person can have a degree of sensitivity to all these things,
but not all may be responsible for worsening of their eczema. This can make the
results difficult to interpret.
It is most useful for children under five, particularly where
there is a history of intolerance to certain foods. Avoidance of these foods
can lead to a marked improvement of their eczema.
Prick testing
This is a specialist test used for the investigation of
immediate (Type 1) sensitivity to things which come into contact with the skin.
This sensitivity usually appears as a sudden onset of itchy 'hives'
on the skin, sometimes with swelling around the mouth and
wheeziness.
The test is used for the investigation of latex allergy. Latex
is mainly found in thin rubber gloves used in hospitals or by dentists, and is
also found in balloons and condoms.
The test consists of putting a number of substances on the arm
in various strengths and introducing them into the skin with a very fine
needle. After approximately 10 minutes, each area tested is assessed to see
whether a red patch has developed.
Patch testing
This is used in the investigation of
allergic contact dermatitis. It is
mainly used where eczema is confined to a specific site, for example, only the
hands or the face.
Eczema which is worsening or failing to respond to conventional
treatment should also be investigated in this way. A person’s occupation is
important in determining which substances to test for.
Patients for the patch testing clinic are usually asked to come
along for three days in one week: Monday, Wednesday and Friday.
First visit
This is when the consultant decides which tests to
use.
Patients are also asked to bring along products that they use
regularly and feel may be aggravating their skin, because often these can be
tested as well.
The substances are stuck on to the back, and the area must be
kept dry for two days.
Second visit
On the second visit the patches are removed. Often, the
consultant will have a first look for any reactions.
Third visit
On the third visit, the patch tests are formally read, because
it takes about four days for a reaction to show up.
A red, raised itchy patch indicates an allergy to a particular
substance. The consultant can then give information about products that contain
this substance (allergen) and explain how to avoid them.
Unfortunately, avoidance of a proven allergen doesn’t always
completely clear the eczema, particularly if it is on the hands.
Negative patch tests are helpful because they can make a
diagnosis of
irritant contact eczema more likely,
and it can be treated accordingly.
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Last updated 15.09.2005
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