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Three thousand subjects in North America were patch‐tested with 19 allergens. The most frequent sensitizers observed include nickel sulfate, caine mixture, potassium dichromate, balsam of Peru, thimerosal, ethylenediamine hydrochloride, paraphenylenediamine and thiram. The prevalence data compared with an earlier study indicates that the initial allergens reported warrant their continued inclusion in a routine patch test screening series in North America.

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The patch test is essential for the definitive diagnosis and the competent management of allergic contact dermatitis. It can only be effectively studied by the accurate evaluation of this test. Despite the fact that the patch test does not duplicate the multiple applications, sweating and maceration of clinical exposure, it is safer and quicker for the identification of allergic contact dermatitis than trial and error or avoidance and re-exposure. Reliable in vitro tests may eventually be developed for this type of hypersitivity, but today the patch test is indispensable for proving the cause and directing the treatment of this frequently encountered clinical problem.The patch test is so well known that the term is widely used by the public. The basic principles on which the patch test are based are simple, and the execution of the test seems straightforward. Nonetheless, training and experience in the performance and interpretation of the test are necessary if the results obtained are to be valid. Patch tests have frequently been performed by persons lacking this necessary background, and the errors that have resulted have tarnished the image of the procedure, so that many dermatologists came to doubt the value of the patch test in their practices.Address for reprints: Norman B. Kanof, M.D.,