Allergic Contact Dermatitis and Patch Testing

Allergic contact dermatitis is a common cause of an itchy rash. Sometimes the rash is diffuse extending all of the body, but other times it only affects certain areas such as the eyelids or the hands. An eyelid rash or hand rash should be assumed allergic contact dermatitis until proven otherwise. A biopsy of the rash will show this diagnosis to be accurate, but that is only the first step in the process.

The real solution on suspected or known allergic contact dermatitis (ACD) is to perform patch testing. Patch testing is a unique type of allergy test, usually limited to the field of dermatology. As the ACD is a delayed type IV allergic response, the testing for this type of reaction is also delayed in nature. Patch testing involved places sticky patches on the back of the patient (if there is no existing rash present on that site), and having them keep those in place for 48 hours before returning to the clinic for the readout of the patches. In our dermatology office, we test for a total of 80 allergens ranging from preservatives found in grooming products to fragrances to rubber accelerents to metals. These allergens are selected by a panel of dermatologists who specialize in allergic contact dermatitis, and are the most commonly reactive allergens found in manufactured goods in North America. Other countries and regions may use varied patch tests based on manufacturers in those areas.

Many patients ask me how they could suddenly start having issues to products they have always used. They state they have not changed anything at all in their routine, but yet, the rash is present. I remind them that the immune system, which dictates when allergies may begin, is ever-changing and evolving. Sometimes, errors within protein manufacture occur in the human body, and one can end up with an allergy to something they have never had issues with previously. Other times, the manufacturer of a particular product which a patient has used without issue in the past is changed–perhaps one ingredient is changed without the consumer being aware of that change. Now the product is technically a completely different entity and problems with ACD can occur.

When it is a definitive diagnosis of eyelid dermatitis, hand dermatitis, or biopsy proven ACD on body, it is absolutely necessary to perform patch testing in order to eliminate the allergen inducing the rash, and prevent unnecessary use of medications such as steroids to chronically treat ACD.

Without patch testing, it is impossible to determine through process of elimination what is driving the reaction. I liken it to bringing your car in to a mechanic and asking them to fix an issue, but telling the mechanic he won’t be able to look under the hood at all. Due to the delayed nature of the rash that is ACD, it is impossible to know what is causing the reaction without this imperative test. ACD is very different from an immediate type I allergic response. In cases where someone has a type I reaction, they immediately know that what they have just ingested is what is causing a reaction. This is a completely different mechanism from the delayed type IV reaction seen in allergic contact dermatitis.

Once we have completed patch testing, we can now give the appropriate information to the patient on how to avoid the allergen(s) that are causing issue. Information such as the name of the allergen, the multiple other names that the allergen may be listed by on packaging, as well as product categories that the ingredient may be contained within. Once the information from patch testing is determined, it is now up to the patient to eliminate and avoid all known contact allergens. If testing does not reveal answers as to the cause, it may simply be that the allergen that patient is allergic to is not one of the 80 most common as determined by the Allergic Contact Dermatitis Society.

There are approximately 3000 allergens out there, that patients are exposed to on a regular basis. Testing to all of those does not make financial sense. If a patient falls into this less common category, dermatology treatments are still available, but would not include chronic topical or oral steroid use, rather opting for safer long-term treatment in a patient whose cause, despite appropriate testing, may be unknown.