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Author: Dr Emily Ryder, Dermatology Registrar, Waikato Hospital, Hamilton, New Zealand; Chief Editor: A/Prof Amanda Oakley, July 2014.
The open application test is also sometimes called the “repeat open application test” or abbreviated as ROAT. It is a simple method of testing for allergic contact dermatitis (delayed-type allergy or type-4 hypersensitivity reaction).
The product to be tested by open application is applied directly to a small area of skin. The application is repeated on several occasions and the treated area observed to see whether contact dermatitis arises.
The principal is similar to patch tests; however, open application testing may be done by the patient.
It may also be helpful after patch testing if there is an equivocal or weak positive result or to determine whether a mildly positive result is clinically relevant.
Open application tests may also be helpful for people with a tendency for contact dermatitis, to test a new product before using it on a wider area.
Open application test is not used for type-1 immediate hypersensitivity reactions, such as hay-fever, allergic forms of urticaria, allergies to consumed foods or anaphylaxis, when prick tests are more suitable.
Products that are known to cause irritant contact dermatitis (such as household detergents) should not be tested in this way.
For an accurate result:
If the treated skin continues to look and feel normal by the end of the week of testing, the person is unlikely to have a significant allergy to the substance tested.
Conversely, if the individual develops a patch of red skin, dry skin, itching or frank dermatitis at the site of application, there may be an allergy to the substance or to one of its ingredients.
Immediate signs of skin irritation are usually due to an irritant contact dermatitis, rather than an allergic contact dermatitis. In contrast, an allergic reaction is often only evident several days after applying the substance.
Open application tests may result in adverse events. For example:
Hair dye allergy is common and can be serious. Most hair dyes have specific instructions for testing for allergy on the packet. Allergy testing should ideally be carried out at least 48–72 hours before every use, as an allergy may develop only after repeated use.
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