Patch Testing

Abstract

Patch testing is the main investigation in the diagnosis of allergic contact dermatitis. It reproduces in miniature the clinical expression and the pathogenic mechanism of allergic contact dermatitis. It may seem simple to apply and read but in actual fact, the procedure is fairly complicated and proper performance requires adequate experience. Apart from in subjects with eczematous and noneczematous contact dermatitis, patch tests should be done in all cases of other eczematous dermatoses, and also in all cases of worsening of preexisting other dermatoses when a superimposed contact allergy is suspected due to topical treatment, for example. As regards the patch test procedures (selection of materials, type of chamber, dosing of chambers, sites of application, occlusion time, some practical suggestions, and reading times), there is a vast pertinent literature, and since there are various national legal regulations governing the execution of patch tests, dermatologists should be aware of the national frameworks in their own country. The readings of patch test reactions must be done by a dermatologist with adequate experience, in order to discriminate irritant and doubtful reactions and distinguish between doubtful and weak positive reactions. In particular the execution of tests with the patient’s own materials requiries proper experience and a highly trained staff. The various potential adverse effects (active sensitization, flare-up of contact dermatitis, Koebner phenomenon, persistent reaction, “angry back”, necrosis, scarrings, anaphylactoid reactions) must be regarded as complications not risks of patch tests, and therefore should not exclude their use. In order to be able to offer the patient useful prevention norms, an extremely important point is the assessment of the clinical relevance of the patch test reactions.