Patch testing is a technique used to see if you are allergic to a particular substance or set of substances. It's for those with a delayed reaction that takes a couple of days to emerge. Tiny amounts are placed on your skin and you wait to see if you get a reaction. A positive test is where you get redness and bumps in the area exposed to the allergy-causing substance. It takes an expert to interpret the results, as it’s not straightforward. And it helps if you have your own suspicions of what’s making your skin react, so tests can be tailored to you. It’s used for those with eczema or contact dermatitis if it’s proving difficult to control with medications, in which case an allergen may be suspected. It’s also useful if you suspect something at work may be causing a rash, as your employer can hopefully offer measures to help. Identifying an allergen can make a huge difference in avoiding it and getting yourself better. Eliminating other potential allergens can also be very important to someone’s quality of life. It's a commitment of your time - several days and clinic trips and no showering during the days of testing. There's a quicker test called the skin prick test, and blood testing, called RAST IgE, but these are for an immediate reaction, so results are ready within half an hour. This is for allergens more likely to cause hives (urticaria) and hay fever-type symptoms than contact dermatitis or eczema rashes, which is a delayed hypersensitivity reaction.
There’s a standard set of tests for the most common allergens to cause allergic contact dermatitis, called the European Standard Battery. This includes nickel, chrome, hair dye, and certain resins, plants and fragrances. If you have a specific concern or think it’s work-related, there are other sets of tests – for example, the florist series for common plant allergens, the hairdresser series, the adhesives series, the fragrance series. There’s even one to test allergies to common medicated creams, called the steroids and medicaments series.
Allergens in small metal discs are put on your back and taped in place. It’s a commitment, as you’ll have to leave them on for at least 72 hours. You return to the clinic after 2 days and 3 or 4 days, to have any reaction inspected. Some reactions come on quite quickly, others appear later. You can’t shower or go swimming while the units are taped to your back, which is worth bearing in mind for planning. It’s important to ensure that you have no eczema or dermatitis on your back at the time of testing, otherwise results are difficult to interpret. Your specialist can treat this as needed before testing starts.
Such a small amount of potential allergen is used in the testing process that the risk is very low. The reaction is usually mild and confined to a small area, but there’s a possibility you have a bigger reaction, especially if you test positive to multiple substances. There’s a medical adage that goes, if you test for something, you should be ready for the answer. Tests don’t always confirm what you think – a substance you believe is definitely causing your rash may not actually give a positive result. And something you thought you were fine with may actually prove reactive in testing. How do you interpret this? That’s for your specialist to discuss with you, but generally, your own experience usually trumps allergy testing results.
There are private clinics that offer patch testing – dermatologists or allergists are the medical specialists for this area of expertise, and it does require skill to test for the right things and interpret results. They can determine an irritation from a true allergy, and advise on the best action plan. Other non-doctor private clinics offer patch testing and other types of allergy testing. Your GP may consider referring you for patch testing or to a dermatologist in certain circumstances. This is if your eczema has suddenly got worse, or if you have hand, face or genital skin rashes that are recurrent or resistant to standard treatment. There's often a bit of a wait for this on the NHS. Your GP may refer you for work-related dermatitis, but if your employer has private cover or access to an occupational medicine doctor, this may be a good place to start and will be much quicker. And any action plan can be implemented directly in the workplace.