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Patch testing for skin allergies |
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Written by Dr Virginia Hubbard, specialist registrar in dermatology and Dr Malcolm Rustin, consultant dermatologist
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What is patch testing?
Patch testing is a way of identifying whether a substance that
comes in contact with the skin is causing inflammation of the skin (contact dermatitis).
There are two types of contact dermatitis:
irritant contact
dermatitis and
allergic contact
dermatitis .
Irritant contact dermatitis
An irritant substance is one that would cause inflammation in
almost every individual if it was applied in sufficiently high concentration
for long enough. An irritant reaction is caused by the direct contact of an
irritant substance with the skin and does not involve the immune
system.
Allergic contact dermatitis
An allergic reaction is specific to the individual and to a
substance (or a group of related substances) called an allergen. Allergy is a
hypersensitivity (oversensitivity) to a particular substance, and always
involves the immune system. All areas of skin that are in contact with the
allergen develop the rash. The rash will disappear if you avoid contact with
the substance.
Patch testing can help to differentiate between the two. The
test involves the application of various test substances to the skin under
adhesive tape that are then left in place for 48 hours. The skin is then
examined a further 48 hours later for any response. This can help the doctor
decide which allergens you are allergic to and identify those that could be
aggravating your dermatitis. The doctor will then be able to advise how you can
avoid the allergens.
Why is patch testing done?
If you have a dermatitis that started recently or if you have a
persistent or unusual eczema, your dermatologist may suspect you have an
allergic contact dermatitis. If you have been using a medication on the lower
legs, hands, face, ears, eyes, anal or genital region for a while and
subsequently developed dermatitis, your doctor may suspect that this is
aggravating the dermatitis.
Patch testing is the only way that your doctor can prove that a
substance is causing or aggravating your dermatitis. Once an allergen is
identified, avoiding it should help cure your dermatitis.
How is patch testing done?
First your doctor will discuss your skin problem with you.
Subjects discussed include:
-
the site where your rash began and how it developed.
-
the treatments you have tried.
-
previous skin disease.
-
the general health of your and your family, especially any
tendency to get one or a combination of asthma, hay fever or eczema.
-
cosmetics and toiletries used.
-
your occupation - this will focus on materials used at work
and the effect of weekends and holidays on your dermatitis (if it settles
during these times, it is likely that you are in contact with an allergen at
work). If other workers are affected with a similar rash then tell your doctor.
-
your hobbies.
If you can think of anything that you were in contact with
around the time the rash first appeared then tell your doctor. Do not assume
that just because you have been using something previously without a problem,
it will not be the cause. Sometimes a cosmetic that you have been using for
some time can become the cause of dermatitis.
Your doctor will then examine your skin. The dermatitis is
usually most severe at the site of exposure, but can be widespread (for
example, if a patient with an allergy to a substance in nail varnish touches
her face, the dermatitis may spread).
Which allergens are tested?
Your dermatologist will suggest which allergens you should be
tested for. The standard selection of allergens used is the European Standard
Battery, which consists of the commonest allergens. Together these cause 85 per
cent of all allergic skin reactions. In addition, the dermatologist may suggest
additional patch tests using other allergens specific to your occupation or
site of the rash as well as your own cosmetics.
Patch testing
Patch testing should be done on a skin site where the dermatitis
is not apparent. The allergens are mixed with a non-allergic material (base) to
a suitable concentration. They are then placed in direct contact with the skin,
usually on the upper back, within small aluminium discs. Adhesive tape is used
to fix them in place, and the test sites are marked. The patches are left in
place for 48 hours, during which time it is important not to wash the area or
play vigorous sport because if the adhesive tapes peel off the process will
have to be repeated.
The patches should not be exposed to sunlight or other sources
of ultraviolet (UV) light. After 48 hours the patches are removed and an
initial reading is taken one hour later. The final reading is taken a further
48 hours later. Additional readings beyond 48 hours increase the chance of a
positive test patch by 34 per cent. The patient should refrain from washing
until the last reading is taken.
Interpretation of results
Any reaction seen is scored according to the International
Contact Dermatitis Research Group system, as follows:
-
+? = doubtful reaction: mild redness only.
-
+ = weak, positive reaction: red and slightly thickened skin.
-
++ = strong positive reaction: red, swollen skin with
individual small water blisters.
-
+++ = extreme positive reaction: intense redness and swelling
with coalesced large blisters or spreading reaction.
-
IR = irritant reaction. Red skin improves once patch is
removed.
-
NT = not tested.
The distinction between allergic and irritant reactions is of
major importance. An irritant reaction is most prominent immediately after the
patch is removed and fades over the next day. An allergic reaction takes a few
days to develop, so is more prominent on day five than when the patch is
removed.
A substance that causes an irritant reaction may exacerbate any
underlying dermatitis such as atopic eczema, but this will not get worse with
time and can be prevented by wearing a barrier cream or plenty of moisturiser.
A substance that causes an allergic reaction will cause a
dermatitis and should be avoided completely if possible. The more times the
skin is exposed to the substance, the worse the allergic reaction can
become.
Photo-patch testing
Some chemicals produce an allergic reaction only when exposed to
light (usually ultraviolet type A light, UVA). Patients who are oversensitive
to light and those with a rash that appears on parts of the body normally
exposed to light (mostly the face, the 'V' of the neck and the hands)
but that does not appear in areas shielded from the light (eg under the chin
and the triangle between the nose and the mouth) should have a photo-patch
test.
With photo-patch testing, two identical sets of allergens are
applied to the back on day one. One of the sets is exposed to UVA light, and
the sites are then examined as usual. A positive photo-patch test is recorded
when an allergic reaction appears only on the light-exposed site.
What should I do if a reaction is positive?
-
You should be given detailed information about sources of the
allergen.
-
Scrupulously avoid any further contact with the
allergen.
-
Carefully read ingredients of new products, especially
cosmetics.
-
Use barrier creams and protective clothing to avoid the
allergen.
-
Use alternative products that do not contain the allergen.
-
If the allergen is at work then discuss the options with your
employer. They should provide materials to protect you from the allergen or if
this is not possible, consider how to change your work.
Common allergens tested
-
Balsam of Peru: an aromatic mixture made from resins and
essential oils. It is found in the haemorrhoid preparation
Anusol, some
perfumes and certain spices.
-
Caine mix: local anaesthetics found in preparations for sore
throats, sunburn remedies, haemorrhoid preparations, Wasp-eze. Used by dentists
and doctors for minor surgical procedures.
-
Carba mix: rubber 'accelerators' (chemicals used to
speed up the polymerisation process in the manufacture of rubber). It is found
in rubber gloves, shoes, bandages and elastic. Of those allergice to carba, 85
per cent are also allergic to thiuram.
-
Chlorocresol: a substituted phenol preservative that kills
bacteria. It is widely used in medications and some cosmetics. It cross-reacts
with Dettol, which you should also avoid if you have a chlorocresol
allergy.
-
Chromate: a metal used for plating other metals to prevent
rusting and in the manufacture of stainless steel. It is also found in cement
and tanned leather.
-
Cobalt: found in jewellery, dental implants, artificial
joints, jet engines. Most patients are also allergic to nickel, and some are
also allergic to chromate.
-
Colophony: present in adhesives, plasters, paper, printing
inks, medicated creams, glue tackifiers (stamps, labels), and
cosmetics.
-
Epoxy resin: plastics, used mainly as adhesives in the
industrial setting but also by DIY enthusiasts. Found in two-component glues,
such as Araldite.
-
Formaldehyde: preservative frequently used in household
products and in industry. Often found in cosmetics and shampoo.
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Fragrance mix: used in patch testing, this collection of eight
individual fragrances detects about 75 per cent of patients allergic to
perfume. If you have perfume allergy, you will not be allergic to all
fragrances, but you cannot tell from the label which fragranced cosmetics are
safe. Avoid all cosmetics listing 'parfum' as an ingredient on the
label. Also found in air fresheners, washing powders and candles.
-
Lanolin: produced by sheep to protect the fleece from the
results of weathering. It is widely used in cosmetics, medical creams and
bandages.
-
Mercapto mix/thiazoles: a rubber accelerator found in rubber
shoes, insoles, gloves and elastic. It is also a component of balloons and
bandages.
-
MBT (mercaptobenzothiazole): another rubber
accelerator.
-
Neomycin: an antibiotic commonly used in ear and eye drops and
creams to treat infected skin problems. Cross-reacts with other
antibiotics.
-
Nickel: 10 per cent of women and at least 1 per cent of men
are affected by nickel allergy. Nickel is released from metals such as alloys
or electroplated items. Found in jewellery, keys, coins, zips and buckles,
pacemakers and batteries.
-
Parabens: preservatives found in cosmetics and topical medical
products to inhibit the growth of fungi and prevent slow deterioration. They
are commonly used in cosmetics, household products, glue, shoe polish, shampoos
and conditioners, sunscreens and medical creams.
-
PPD (paraphenylenediamine): a permanent hair dye that is very
frequently used in hair salons and at home. Dyed hair cannot cause an allergy
but the dye may do during application. Also found in skin paints and
occasionally in fur and leather dyes.
-
Primin: a substance produced by the plant Primula obconica, a
common houseplant.
-
PTBPF resin (para-tertiary-butylphenol-formaldehyde): a
synthetic polymer used as an adhesive. It is often combined with leather or
rubber to make shoes, handbags, watchstraps, hats and belts.
-
Thiuram (tetramethylthiuram disulphide or TMTD): another
rubber accelerator that is also found in pesticides. People who are carba
allergic often react to thiuram. Patients who react to the drug disulfiram (Antabuse),
used for alcohol dependence, may also be allergic to thiuram.
-
Toluene sulphonamide formaldehyde resin (TSF resin): the
commonest polymer in nail polish and a frequent allergen.
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Last updated 05.10.2005
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