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  • The London Skin and Hair Clinic
    233 High Holborn
    London WC1V 7DN
    020 7183 4565
    233 High Holborn, London WC1V 7DN
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    Patch testing

    Patch testing can help identify substances which patients are allergic to.

    What is a Patch testing

    If you are experiencing skin conditions which are triggered by contact with certain substances, patch testing may be helpful.

    What is involved in a Patch testing

    A Dermatologist or nurse will apply tiny quantities of substances  to the upper back using individual square plastic or round aluminium chambers. These are secured in place with hypoallergenic adhesive tape and left for 48 hours. Typically the patches are applied on a Monday, then removed on a Wednesday when a reading is taken, and then a further reading is taken on a Friday, two days after the patches were removed. The first appointment will take around 30 minutes.

    We use the T.R.U.E patch test series which reflect the most common and important causes of dermatitis. Common allergens included in the baseline series include chemicals found in hair dye, fragrances, adhesive, rubber and metals such as cobalt and nickel. The 36 patches cover 58 allergens.

    Two days later the patches will be removed, and the dermatologist will make a 48 hour reading of the skin’s reaction to the allergen so far.

    Two days after this, in the third appointment the dermatologist will complete a 96 hour record noting which sites had a positive or negative reaction to the allergen. This will be either; negative, irritant reaction, uncertain, weak positive, strong positive or extreme reaction.

    The Dermatologist will provide patient information leaflets on the allergens that patients test positive for. They will help identify what daily items you are using that may contain these items, so that you can avoid coming into contact with these going forward. In some cases the Dermatologist uses a patch test to rule out certain skin conditions, so they can diagnose appropriate treatment.

    What is the cost of a Patch testing

    You can find our fee schedule here.

    What are the risks or potential side effects of Patch testing?

    Sometimes nearly all the test sites will become red and itchy leading to inconclusive and misleading results, as the allergen cause is then difficult to identify. This is most likely to occur when the patient has very active dermatitis and is known as a false positive result.

    At the opposite side of the scale there may be little or no apparent reaction to the substance which is causing the dermatitis (false negative result). In these cases retesting may be required sometimes one allergen at a time.

    When the patch test is positive occasionally the reaction may persist for several weeks and require treatment with a topical steroid.

    Rarely, the skin may react to a new allergen as the result of the test, this would usually occur around 10 days after the test was applied.

    Is there any preparation needed before having Patch testing?

    You will require a consultation with a dermatologist prior to the patch test who will review your medical history and look to identify potential allergens that should be tested for. You may also be asked to bring some of these substances along with you such as regular cosmetics you use (including make –up, moisturiser, sunscreen), clothing (only around 1cm required), identified food items, and creams and ointments. Only very small quantities are required.  If you work with industrial chemicals you may be asked to provide a data sheet for these, and to send some in one week prior to the appointment.

    The back, where the patches will go, should be protected from sun exposure for 4 weeks before the tests. Wear old clothing to the appointments, as marker pen can stain.

    What aftercare is required for Patch testing?

    After the patch test is applied it is advised that you keep the back dry so no baths, showers, swimming or strenuous exercise which may cause unnecessary sweating. Care should also be taken to avoid rubbing the patches off.

    If the reaction to the patch test is extreme and causes blisters, some after care may be required such as washing and dressing the site of the blister.

    Reactions to positive results may persist for several weeks and therefore may require topical steroid cream to ease the inflammation.

    In most cases the skin will return to normal within a matter of days.

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