Immediate contact allergy has to be differentiated from non-immunological contact urticaria. Non-immunological contact urticaria is less serious, and its localization is not necessarily reproducible. In fact, the challenge is to keep in mind that allergic etiology is possible and, thus, testing is helpful. Testing is performed with suspected products and then, if necessary, with the product’s ingredients. The product is tested first by an open test, then if negative by a closed open test, and then by prick tests. The test is considered positive if the Lewis triad (erythema, edema, and pruritus) is detected and if other people do not react when they are tested with the same product in the same conditions. Some specific IgE can be sampled in the blood but is not always found, and sometimes Western blot analysis is needed [6, 7].
Allergic contact dermatitis should be suspected when children have persistent eczema, even at an early age. Before the age of 3 years, reactions in children have to be considered atopic unless the localization is typical, such as diaper dermatitis, isolated foot dermatitis, asymmetric periflexural exanthema, or umbilical dermatitis. Atopic dermatitis should also be considered if symptoms in children appear from spring to autumn or in cases of lack of efficacy of a treatment. In children over 3 years of age, allergic contact dermatitis should be suspected for the same reasons. Moreover, the possibility of allergic contact dermatitis has to be taken into account in children with no history of atopy or with recently diagnosed dermatitis. There is no particular clinical characteristic of allergic contact dermatitis due to fragrances or cosmetic products in children other than what is known in adults; it is of huge interest to obtain a detailed anamnesis in order to specify the environment of the child and all his contact regarding fragrances and cosmetic products. However, children themselves may surreptitiously use some cosmetic products, such as their mother’s cosmetic products. Therefore, the diagnosis may not always immediately be evoked. Patch testing is performed with either a children’s special baseline series [8, 9] or with the adult baseline series. All products identified in the personal history have to be tested. The main fragrance markers in the baselines series are fragrance mix I 8 % in petrolatum, fragrance mix II 14 % in petrolatum, and Myroxylon pereirae 25 % in petrolatum. Colophonium 20 % in petrolatum can also be considered a fragrance marker. Hydroxymethyl-pentyl-cyclohexene-carboxaldehyde is another frequent allergen in children. Patch tests with these allergens are not known to be irritating in children. Proxy allergic contact dermatitis due to fragrance in children has been reported. The relevance of each positive patch test has to be researched and can be found in the cosmetic product information or in other sources. For the same patient, the relevance can be found in different sources.
In a review in 1997, Conti et al.  found that 44 % of children’s reactions to preservatives, such as formaldehyde releasers, parabens, the methylchloroisothiazolinone Euxyl K400, and the antioxidant butylhydroxyanisole, were atopic. Since 1997, Euxyl K400 has therefore been prohibited in European cosmetic products. Regulation of formaldehyde and formaldehyde releasers has been reinforced, and the public has been informed that parabens are harmful preservatives, so they are rarely used in cosmetic products. The main cosmetic markers in the baseline series are used not only in cosmetic products but also in detergents, food, and industrial products. Therefore, the frequency of contact dermatitis due to cosmetic products cannot be evaluated by the rates of positive patch tests to these allergens without establishing their relevance by reading products labels. Leave-on cosmetic products can be tested by patch tests, and rinse-off cosmetic products have to be tested by open tests. The reading is done on day 2 and on day 3 or 4, according to the International Contact Dermatitis Research Group (ICDRG) guidelines. Positivity to a leave-on cosmetic product has to be confirmed by a repeated open application test, because some irritating reactions can occur behind the patch. When allergic contact dermatitis is due to an identified cosmetic product, the child must be tested with the ingredients to determine the allergen unless it has been identified in the other tests. There are no data on the main cosmetic allergens specifically relevant to children. In adults, the main allergens in cosmetic products are fragrances, preservatives, surfactants, and paraphenylenediamine . Allergy to paraphenylenediamine in children is mainly due to black henna tattoos but can also be due to shoes or to the mothers’ hair dyes by proxy contact. In some selected populations of children, propolis 20 % in petrolatum is a frequent allergen, which can be found in biocosmetic products and in folk medicine . Some sunscreens have been found to be allergens in children. Regularly, allergic contact dermatitis in children is due to new allergens in cosmetic products, such as octocrylene , glucosides or fructooligosaccharides , or chondroitin sulfate . Sometimes, an ancient allergen such as wool alcohol is reactivated because it is used again despite its reputation.