Open patch test and the diagnosis of contact urticaria

Some food substances can induce an immediate urticarial-type reaction at the point of contact. No standardised test exists for investigating such contact urticaria, but one can demonstrate such a reaction by an open test. The substance is placed on the skin of the flexor surface of the forearm for 30-45 minutes in an attempt to replicate the urticaria. It may be necessary to use non-intact, eczematous skin. This contact urticaria may be secondary to an allergic or non-allergic reaction. In the non-allergic type no previous sensitisation has taken place; the individual does not have specific IgE to the substance. The urticaria occurs because of non-immunological release of vasoactive substances in the skin. Substances that may affect the skin by this mechanism include acetic acid, benzoic acid, cinnamic acid, sorbic acid and balsam of Peru. Contact urticaria can also be mediated by allergic mechanisms, chiefly specific IgE mediated. Foods capable of causing a reaction in such sensitised people include milk, eggs, fish, nuts, fruits and vegetables.

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