What is patch testing and what is it used for?
Patch testing is a specialised form of test done by a dermatologist on a person’s skin to see if they have a contact allergy i.e. if their skin is reacting to an otherwise harmless chemical allergen in the outside world. Examples of common allergens that cause skin contact allergy include nickel in metal jewellery, perfumes, hair dye, rubber chemicals and lanolin. There are thousands of recognised allergens and they include synthetic and natural substances.
Patch testing is usually advised when you or your dermatologist suspect that you have a contact allergy or where this needs to be ruled out. It is sometimes done to investigate allergy to an oral or injected drug, but is not a test for food allergy or allergy to inhaled substances such as pollen and animal fur.
Skin contact allergies usually cause dermatitis – a red, itchy rash that lasts for days to weeks or longer. These can be easily overlooked in someone who already had an inbuilt or ‘constitutional’ kind of dermatitis such as atopic eczema.
What does the test involve?
Normal skin – usually on the upper back or upper arms – is tested with small quantities of different allergens applied on adhesive tape (the ‘patches’) to see if they cause a reaction. Most people will be tested with about 50-100 allergens. The patches need to stay in place for two days and they are then removed and the skin is examined to see if there are any allergies. A second examination is carried out two days later and this usually concludes the tests. A positive reaction appears as a small raised, itchy swelling about the size of a finger-nail.
Patch tests usually involve three clinic visits in the same week on a Monday, Wednesday and Friday for application of the allergens and evaluations as above. It is important to keep the tested area of skin dry throughout this period.
Are there any risks?
Patch testing is a long-established and safe test. A small number of people may have a strong allergic reaction that leads to blistering of the skin and this can leave a lasting blemish with altered pigmentation or very rarely, a scar. There is also a small risk of becoming allergic to one of the allergens after the procedure. As the safety of patch tests has not been proven during pregnancy (there are no official trials) they are not usually recommended unless essential. They can otherwise be done in all ages including young children if needed.
When do I get my results?
You will be informed of the results at the end of the procedure, which is usually on the Friday appointment. Sometimes the tests are negative and this can be reassuring as allergies have been ruled out. If you have allergies, you will be given information about where you are likely to come into contact with the allergen and how to avoid this in the future. Your dermatologist will also help advise if the allergy is likely to be the cause of your skin complaint. Skin allergies are usually life-long and cannot be reversed by desensitisation, so the focus of tests is to help you identify your allergies, so you then know what to avoid.