What is patch testing?
Patch testing can help your doctor find out whether your skin
condition is caused by an allergy to substances which come into
contact with your skin, such as products at home, at work or in
leisure activities. You can then be advised of the names of any
identified allergens in order to help you to avoid them.
To what will I be tested?
You will be tested to at least 40 standard substances which are
frequently in contact with the skin, e.g. rubber, preservatives,
metals, perfumes and plants, and also additional substances that
are suspected triggers for your skin problem. You may be tested to
some of your own work or home products, such as personal
toiletries.
What does patch testing involve?
After your initial consultation and assessment with the
consultant, three further planned visits to hospital during a
single week are arranged. On the first visit a specialist nurse
will apply the tests to your back in special small containers
contained within a tape. The containers are identified by marking
your back with ink. Occasionally the arms or the thighs are also
used. Itching of the sites is normal. Allow up to 2 hours for this
first visit.
The substances remain taped in place until your next visit, when
the patches containing them are removed and any reactions noted.
Additional patches may be added, as indicated by the initial patch
test readings. Marking ink or tape will remain on your back and may
be reinforced, to ensure it will persist for a further two days
On the third visit, your back will be examined and any further
reactions discussed with you. Relevant information leaflets will be
provided at this stage. Positive reactions become red and itchy at
the test site and usually become apparent by the third visit (final
reading), although they can occasionally take longer, up to two
weeks.
Negative patch tests can still be helpful since, as far as
possible, contact allergy as a cause of a skin problem will have
been eliminated.
When can patch testing not take place?
You may not be patch tested if you are pregnant, breastfeeding,
have extensive eczema on your back, have a suntan or have used a
sunlamp over the previous 2 weeks, or are on a moderate or high
dose of steroids (e.g. prednisolone tablets more than 10 mg/day).
If you are planning to swim or take strenuous exercise, patch
testing cannot be carried out as the patch tests will fall off. In
exceptionally hot weather patch testing is uncomfortable as normal
showering has to be avoided. A cool shallow bath works well. It is
sensible to wear older garments on the week of patch testing
What side effects may occur?
The skin at the site of a positive patch test normally becomes
red and itchy. The surrounding skin may also itch from the tapes
which are holding the patches in place. Rarely, positive patch
tests may last for a few weeks (usually they go away after a few
days). An increase or decrease in pigment may be seen at the site
of a positive patch test, which may last for months or rarely be
permanent. A positive patch test may be accompanied by a flare of
existing or previous eczema and may rarely become infected.