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Patch testing

Patch testing is a procedure used when investigating allergic contact dermatitis (delayed type allergy).

Allergic contact dermatitis is the manifestation of an allergic response that the body has when coming into contact with certain substances or materials. It might appear as a dermatitis-like rash or groups of red bumps on the skin. It is usually itchy or irritating, with occasional oozing or crusting.

Blisters might appear, there may be significant swelling or scaling, or the skin might just turn red and itchy. These symptoms don’t always appear straightaway; it is often a day or two after contact with the substance when they are actually noticed.

Why would someone need patch testing?

A good example is someone who is working with a particular set of chemicals; such as a hairdresser, a beautician, a machine operator, a cleaner, or someone working in a drug manufacturing facility. They may be developing hand dermatitis or facial swelling related to their workplace.

Patch testing will help in these cases to find out what is causing it, so they can make efforts to avoid such materials or environments.

Another example might be somebody who goes to a massage bar, or to see a beautician for a “facial”, or for a nail treatment, and the following day their face begins to swell up or they develop a rash. The problem might get worse over the ensuing days, affecting their ability to go to work. Their skin may peel as it heals. They will want to know what it was that caused such a reaction, so they can avoid it in future.

The investigation of this kind of problem is very thorough. When patients like this are seen for a consultation, recent activities are explored in detail, often looking at photographs patients have taken, or examining examples of things with which they might have come into contact.

They might bring a 'Material Data Safety Sheet', which is a legal requirement of employers, listing the chemicals to which their workers are exposed. Lists of ingredients of the cosmetics and beauty products they used just before the rash erupted, which they obtain from the manufacturers’ websites, can be very useful in assessing the cause of the problem.

With all this information, an examination of the skin takes place and treatments are prescribed on the day. These might be immediate solutions (to make the problem better straightaway) and ongoing measures. Arranging patch testing, once the skin has settled down, is then discussed.

What happens during patch testing for delayed-type allergy

During patch testing, a patient comes back for three visits within the space of a week. The first visit is on a Friday (Day 0), at which a variety of chemicals (about 150 or more) is placed on their back and arms by a nurse, stuck on with hypoallergenic tape, and then they are requested to keep everything dry for 48 hours. Showering of the back, swimming and sweaty exercise must all be avoided until after the Wednesday (Day 5) appointment, as they might cause the patches to come off.

After 48 hours (Sunday), a companion needs to take photographs of the patient’s back before and after the removal of the patches. Marking tapes are left in place to identify the allergens. The patient comes to see Dr Buckley for reading and consultation on Monday (Day 3). At this appointment, Dr Buckley will see if any itchy areas have developed, or areas of redness, where one or more of the allergens were present. A substance might have to be tested again if anything is unsure.

The patient needs to keep their back and the tapes dry for another couple of days, with no showering, and returns on the Wednesday (Day 5) for the final reading and consultation. If they have had a positive patch test, written information is provided on what the allergen is, where they might find it, and how to avoid it. Patients are informed what alternative products they might use to be safe, and if necessary they receive further prescriptions. General advice and thoughts on skin health are provided.

After that, no further appointments are usually necessary. A few patients choose to come back at a later date if they need further advice, or if symptoms have flared up again, or if they have developed a totally different problem.

What does patch testing test for?

During patch testing, everyone is tested to the 'standard battery' or ‘baseline series’ of allergens, which has around 50 items in it, including metals, rubbers, perfumes, preservatives, steroids, plants and lanolin, covering a wide variety of commonly encountered substances that tend to cause allergic reactions.

The list of allergens in the baseline series changes over time and is decided by the Committee of the British Society for Cutaneous Allergy, on which Dr Buckley sits. Allergens are added and removed as the rates of allergy change.

The top three things to which allergy is found at the moment are nickel, fragrance and preservatives. (Meth)acrylates used in nail cosmetics are also frequent allergens.

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