What is the open application test?
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.
When should the open application test be performed?
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.
Which products are suitable for testing?
Products that are known to cause irritant contact dermatitis (such as household detergents) should not be tested in this way.
For an accurate result:
- Do not test when taking oral antihistamines, oral steroids or other immune modulating medicines
- Do not apply topical steroid creams or calcineurin inhibitors (tacrolimus, pimecrolimus) to the test site for a few days prior to and during the test
How is open application testing carried out?
- Identify a patch of hairless skin where there is no dermatitis and that has not recently been exposed to the sun. A suitable site may be the hairless side of the forearm, the inner bend of the elbow, behind an ear or the side of the neck.
- Apply the product to an area approximately 5 cm by 5 cm.
- If the product is a cleanser or shampoo, wash it off a minute or so later.
- Reapply the product to the same patch of skin twice a day for a week.
- Any dermatitis arising at the test site may be treated with topical steroid and emollients.
How are the results interpreted?
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:
- The test may trigger an itchy or uncomfortable rash in other sites (autoeczematisation)
- A dark patch of skin (hyperpigmentation) may arise at the site of the test and persist for some weeks
- False negative result due to sun exposure on the test site
- Falsely negative result if the substance is applied to skin that is thicker than the skin it is intended for (ie face or eyelids)
- False negative or false positive reaction due to different frequency from the usual way the product is used
Open application testing for hair dyes
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.