Patch testing is useful for the evaluation of allergic contact dermatitis, and may identify allergens not identified by skin prick testing or blood testing. This test is used to identify whether a substance that comes in contact with the skin is causing inflammation of the skin (termed “contact dermatitis”). There are two types of contact dermatitis: irritant contact dermatitis and allergic contact dermatitis. An irritant substance is one that would cause inflammation in almost every individual if it was applied in sufficiently high concentration. Examples of irritants include solvents such as alcohol or xylene, hydrofluoric acid, alkalines, and many others chemicals. Allergic reactions are specific to the person and substances involved, arising from hypersensitivity reactions involving the immune systems response to an allergen (antigen). All areas of skin that are in contact with the allergen may develop the rash, and the rash will disappear if you avoid contact with the allergen.
How is Patch Testing Done?
Patch testing consists of three separate clinic visits, should be done on an area of skin where the rash is not present. During the first visit, small quantities of the suspected allergen are placed in non-allergenic aluminum discs. These are then placed in direct contact with the skin, usually on the upper back, and kept in place with hypoallergenic adhesive tape. The patches should remain in place undisturbed for at least 48 hours. You should avoiding getting the area wet (no shower, swimming, or vigorous exercise) during the time the patch test is occurring.
At the second appointment, usually 48 hours following initial placement, the patches are removed. A preliminary reading is done on any reactions that are apparent.
You will then return 48 hours later for the final reading, at which point positive reactions can be confirmed. In some cases, a more delayed reading after 7 days may be required, especially if metal allergies are suspected.
The chemicals included in the standardized patch test kit are responsible for approximately 85% of allergic contact dermatitis, with the most common offenders being Nickel, Balsam of Peru , and Fragrance Mix I. We also have the ability to test for other specific chemicals or metals in most cases.
Preparing for Patch Testing
Some medications such as steroids (Prednisone, Solu-medrol, etc) and immunosuppressive drugs can adversely affect the results of testing. As such, you should avoid taking oral steroids or immunosuppressive medications for at least a week prior to testing if possible. Inhaled steroids (such as those used for asthma treatment) and nasal steroids are ok to continue and will not affect the results. It is recommended that you avoid topical lotions, steroid creams, or other products prior to your testing. Antihistamines are permissible and can be taken if needed. Avoid sun exposure to your back for two weeks preceding your patch test, as this may suppress positive reactions. You also must not be pregnant or nursing.
What to Expect After Testing?
We will discuss any positive reactions as well as avoidance measures you can take. If you have a positive reaction, you should carefully read the ingredients of any new products, especially in soaps, creams, lotions, and cosmetics, to avoid further contact with the allergen. You can also use barrier ointments such as Vaseline and protective clothing to avoid exposure. If the allergen is present at work, knowing what you are allergic to will help you discuss options with your employer to minimize contact, including using personal protective equipment or possibly a change in work environment.