A patient will often suspect allergic contact urticaria because it starts at the site of skin or mucosal contact within minutes, disappears over about 2 hours and happens again with each re-exposure. However, the diagnosis may not be easy because some people become so sensitive to the allergen that urticaria develops anywhere on the body and, very occasionally, the reaction may progress to anaphylaxis. Non-allergic contact urticaria, by contrast, only occurs at the site of contact and may be delayed. Allergic contact dermatitis may occur anywhere on skin that has contact with the allergen and may become widespread. Like allergic contact urticaria, the rash may spread outside the area of contact. However, it never causes anaphylaxis.
The diagnosis of allergic contact dermatitis may be suspected from its distribution because it starts where the allergen makes contact. Hand dermatitis causes difficulties though because we handle so many potential causes of dermatitis that it can be difficult to know which is important. Allergy can develop months or years after being exposed to a product or a manufacturer can change its chemical composition. It is also commonplace for irritant contact dermatitis to occur as well. Irritant contact dermatitis only occurs on areas of skin that have had repeated contact with the chemical.
The test to confirm allergic contact urticaria is skin prick testing , measuring allergen-specific serum IgE or applying the suspected substance directly to the skin or lip. Patch testing is used to confirm allergic contact dermatitis. Patch testing is a procedure by which many substances (those most usually responsible for this type of allergy plus the ones suspected in a particular case) are placed in the back of the patient with the aim of reproducing a small scale inflammation similar to the eczema the patient presented. The patch tests are left for 48 hours and then they are removed. The final result is assessed after 48 hours more.