Patch testing

Patch Testing
Patient Information Sheet

Allergic Contact Dermatitis
Allergic contact dermatitis is an inflammatory reaction that occurs when substances to which you are allergic come in contact with your skin.
These substances might be an ingredient in your perfume or aftershave, an antibiotic ointment or cream, household cleaners, rubber boots
Whilst the susceptibility to development of contact sensitivity appears to have a genetic involvement, contact allergy itself is never inherited.
Contact allergy is acquired after exposure to the substance (or a cross reactive substance).

What is a patch test?
Patch tests are commonly used to aid in the diagnosis of allergic contact dermatitis. It involves small
amounts of various allergens (the substances you may be allergic to) applied to distinct small squares
on a larger patch which, in most cases, is applied to our back. You will wear these patches for 48
What should I do before a patch test?
Since steroids may suppress a positive test reaction, use of topical steroids on the test site or oral
steroids (equivalent to 10 mg prednisolone) should be discontinued for at least two weeks prior to
Avoid UV exposure on your back for at least four weeks before the patch test as this can increase the
rate of false negatives

What precautions or warnings should I be aware of before having a patch test ?
The substances on the test panels seldom lead to sensitisation. A test reaction that appears later than 7 days after application of the test
may be a sign of contact sensitisation.
A flare-up of your dermatitis may occur if the test is used during an active phase of your condition. Excited skin syndrome (Angry Back) is
a state of hyper-reactivity induced by dermatitis on other parts of the body or by a strong positive skin test reaction. If this occurs, your
doctor may recommend retesting at a later date to determine whether reactions are true positives or not.
A positive test reaction usually disappears within 1-2 weeks. On rare occasions, test reactions may persist for a month, leaving an area of
transient hypopigmentation. Irritation caused by the surgical tape adhesive may also occur, but this usually disappears rapidly.
Are there any special instructions that I must follow while I am wearing a patch test?
Moisture around the test area must be avoided. Therefore, when bathing or showering, you must be careful not to get the test panels or
surrounding area wet. If a test panel does become wet, it may loosen and the test must be repeated. For the same reason, any activity
which may cause you to sweat such as sunbathing or exercising should be avoided.
Itching and burning sensations are common. If this becomes intolerable, it is important that you contact your medical team for advice.
Your back will be marked with a special marker pen to identify the test sites. We recommend that you wear old clothes during the whole
course of the test in case the marker pens stain them.
When will the results be read?
The reactions are usually read after 48 hours and again at 72 and 96 hours after the original application when the allergic reactions are fully
developed and mild irritant reactions have faded.
Neomycin sulphate and p-phenylenediamine, however sometimes cause delayed reactions, which may not appear until 4-5 days after the
application. If this occurs, it is important that you report this to your clinical team. If appropriate, an additional office visit will verify a late
What is the doctor looking for?
Your clinical team will carefully examine the test area for signs of an allergic reaction. This reaction usually consists of rash with swelling,
redness and tiny blisters. Redness alone, however, does not necessarily mean that it is an allergic reaction. If you are allergic, your clinical
team will provide information on where in the everyday surroundings you are likely to come in to contact with the offending substances. You
will also get advice on how best to avoid these substances and your clinical team can also often suggest alternatives to the items that
should be avoided.
How are the results recorded?
The interpretation method used is:
What is important in evaluating a positive test response is not the number of + signs assigned to the test response, but determining whether
the response is a true positive reaction caused by allergy or a non-specific irritant reaction. Your clinical team will advise you regarding this.


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