Patch testing

Patch testing is an investigation we use when looking for allergic contact dermatitis (ACD).

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 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 sometimes be significant swelling, or the skin might just turn red and itchy. These symptoms don’t always appear straight away; it could be a day or two after contact when they are actually noticed. But they are usually quite unpleasant, and something with which most people would want help.

So if someone has had a reaction on their skin that they suspect is due to an allergy, they would come to me.

Why would someone need patch testing?

A good example would be someone who’s working with a particular set of chemicals; maybe a hairdresser, or a printer, or someone working in a nail bar or a drug manufacturing facility. They might find that they are getting dermatitis or that their face is swelling up, and they think “I must be allergic to something that is in this 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”, and the following day their face begins to swell up. The problem might get worse over the ensuing days, affecting their ability to go to work. Perhaps their skin may peel as it heals. They would 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. Patients like this come to me and we explore their recent activities, often looking at photographs they 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 have 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, I would examine them and prescribe treatments, which might be immediate solutions (to make the problem better straight away) and ongoing measures. We would then consider moving on to patch testing, once the skin has settled down.

What happens during patch testing for allergies

At this point the patient would come back for three visits within the space of a week. The first visit would be on a Friday (Day 0), at which a variety of chemicals (about 150) are placed on their back and arms by a nurse, stuck on with hypoallergenic tape, and they are requested to keep everything dry for 48 hours. That means no showering of the back, no swimming or sweaty exercise - these might cause the patches to come off.

After 48 hours, the patches can be removed, but marking tapes are left in place to identify the allergens, and the patient will come and see me again for a reading on the Monday (Day 3). At this appointment, we will see if any itchy areas have developed, or areas of redness, where one or more of the allergens were present. We might have to test a substance again if anything is unsure.

The patient will then go away and keep their back and the tapes. dry again for another couple of days - no showering - and come back on the Wednesday (Day 5) for the final reading. If they have reacted to something, I provide written information on what the allergen is, where they might find it, and how to avoid it. I tell them what alternative products they might use to be safe, and if necessary write further prescriptions. We also go through some general advice and thoughts on skin health together, to address any concerns about dermatitis.

After that, I usually don’t need to see patients again, although a few do choose to come back at a later date if they need further advice, or if symptoms have flared up again, or if they’ve developed a totally different problem.

What does patch testing test for?

During patch testing, everyone is tested for what we call the 'standard battery' or baseline series of allergens, which has around 50 things in it - this includes things like metals, rubbers, perfumes, preservatives, steroids, plants, lanolin - quite 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 British Society of Cutaneous Allergy, of which I’m the President.

For example, in 2018 we added acrylates, an ingredient in some nail treatments. We found that many people were going to nail bars and ending up with dermatitis on their face or other body sites, as their hands came into contact with these sites. Acrylates can also diffuse as a vapour around the room - another potential method of contact – and can then cause a widespread rash. We also remove things from the baseline series when they are no longer relevant - for example a popular ingredient in prescribed creams in the past, called ethylenediamine, isn’t used anymore, so we don’t commonly test for it these days.

The top three things to which we see adverse reactions at the moment are nickel, fragrance and preservatives.