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.
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.
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
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
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.
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
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 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
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
+ = weak, positive reaction:
red and slightly thickened skin.
++ = strong positive
reaction: red, swollen skin with individual small water
+++ = 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
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.
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.
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
Chlorocreosol: 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
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
frequently used in household products and in industry.
Often found in cosmetics and shampoo.
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
another rubber accelerator.
Neomycin: an antibiotic
commonly used in ear and eye drops and creams to treat
infected skin problems. Cross-reacts with other
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
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.
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, used for alcohol dependence, may also be
allergic to thiuram.
formaldehyde resin (TSF resin): the commonest polymer in
nail polish and a frequent allergen.