Although most patients are overinvestigated, the history may offer clues to food-additive reactivity or urticarial vasculitis. In addition, the history may point to physical urticarias triggered, for instance, by pressure or heat. Recent evidence implicates histamine-releasing autoantibodies in at least 25% of patients with chronic idiopathic disease.
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Kanwar et al. (1996) studied this question.