In the last few years, several investigators have detected a high prevalence of positive patch test reactions to palladium chloride in nickel-sensitized patients. In the majority of these patients, the positive patch tests to palladium chloride had no clinical relavance. The purpose of this study was to evaluate the prevalence and the clinical relevance of the patch test reactions to palladium in patients undergoing patch tests. Two thousand three hundred consecutive patients with allergic contact dermatitis were patch tested with PdCl2 (1% petrolatum) between January 1991 and September 1992. A positive reaction to palladium chloride was detected in 171 of the 2,300 patients (7.4%). One hundred sixty-nine of these 171 patients had a concomitant positive reaction to nickel sulfate. In our opinion, the similarity of the external electronic structure of these two elements is not sufficient by itself to justify a wrong immunologic recognition, whereas a stereochemical analysis of the compound of these two metals could better explain the possible mechanism of this cross-reaction.
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