Atopic dermatitis: is allergy assessment necessary for skin?

Dr Michèle Sayag, Allergist - France.

Among patients with atopic dermatitis (AD), an allergy assessment is not a necessity: it isn't useful either for diagnosing the disease or for treating the patient in an effective way. However, it may be necessary in certain circumstances, when there is the possibility, for example, that allergic factors are aggravating the eczema.” Dr Sayag outlines for us the different tests and when to take them.

Dr Michèle Sayag, Allergist - France.

What are the different allergy tests?

Four types of tests can be administered based on the situation. Dr Sayag explains the procedures and objectives for each below.

Skin prick tests: A drop of allergen is placed on the skin on the forearm, and a lancet is then used to prick the skin and make the allergen penetrate. This test explores immediate allergic reaction: results are visible 15 to 20 minutes later. A raised, red bump (a wheal) indicates a positive test. The allergens that are tested are airborne (present in the atmosphere: dust mites, animal hair, pollen, mould) or are food (cow milk, egg white, peanut, hazelnut, fish, stone fruit…).
Measuring E immunoglobulines (IgE) in blood. These are specific antibodies that develop in response to certain respiratory and food allergies.
An oral food challenge, administered only at the hospital (for safety reasons), consists of giving progressively larger doses of a food suspected of provoking an allergic reaction. This is the only way to firmly ascertain a food allergy's authenticity.
Patch tests are performed when a contact allergy is suspected. These tests explore a delayed allergic reaction. The European Standard Series, as defined by the International Contact Dermatitis Research Group (ICDRG), is tested first: about 30 allergens that are most often responsible in contact allergies (metals like nickel, perfumes, preservatives, medicine, cosmetic ingredients such as Peru balsam or lanolin…), followed by specific tests afterwards, if needed. Tests are applied under occlusion for 48 to 72 hours. If miniature eczema appears, the test is positive, and results are reported at the end of the 48 to 72 hours. In a recent study (2018), the allergens that are most frequently at issue among atopic children are nickel, methylchoroisothiazolinone and cocamidopropyl betaine. Piercing an atopic child's ears may be what provokes a nickel allergy.

As Dr Sayag notes, test results must be carefully interpreted. “A positive test speaks of being sensitive to an allergen, yet may not necessarily correspond to a true allergy. A positive test's clinical relevance must always be checked. When a positive test is found to be relevant, it is possible to expect an improvement in the skin's state through allergen avoidance. However, the level of improvement in the skin varies according to clinical signs and the type of allergen.”

A positive test speaks of being sensitive to an allergen, yet may not necessarily correspond to a true allergy

In which cases is allergy exploration recommended?

It isn't necessary to do an allergy assessment on all atopic children, confirms Dr Sayag. Here, she details the three situations where the test makes sense.

When atopic eczema doesn't get better, or gets worse, despite treating properly with an adapted local anti-inflammatory product

When the child with atopic eczema fails to thrive (broken or retarded body-mass index curve)

When eczema is also accompanied by: signs that allude to a food allergy (consumed food or skin contact) ; respiratory (asthma, rhinitis) or digestive (vomiting, diarrhea) signs ; signs alluding to a contact allergy (in specific locations: lips, hands, feet, face)

Key points to remember

There is no need for systematic allergy assessment with atopic dermatitis. An assessment is only necessary in certain, specific cases.
A complete allergy assessment can include four types of different tests.
A positive test can show being sensitive to an allergen, without there necessarily being an allergy.