ABSTRACT
Patch testing constitutes the most important diagnostic and investigative method currently available for studying allergic contact dermatitis (ACD) in clinical practice. The procedure involves the epicutaneous application of a specifi c substance (allergen) that should induce a cutaneous infl ammatory reaction in the susceptible (sensitized) person, while causing no reaction in a nonsensitized person. The local reaction, reproducing the dermatitis “in miniature,” provides a visible representation of the subject’s general ability to react to the substance. Therefore, patch testing employs the agent that causes the disease; it applies that agent to the target organ, and it reproduces locally the pathogenic and immunologic mechanisms and morphological changes of the disease itself. However, as a bioassay, patch testing still confronts several inherent methodological problems and requires strict observation of the technical aspects and critical assessment of the results. The issue of whether a positive patch-test reaction is causally linked to the disease being studied involves several
pitfalls including the inherent risk of false-positive responses and the diffi culties in assessing clinical relevance. Besides, the magnitude of the problem of false-negative results is largely unknown. These issues are scarcely mentioned in the literature and frequently overlooked in clinical studies on series of ACD patients. Recognizing the benefi ts of patch testing as well as all its possible pitfalls is of practical importance to the physician using this method for clinical diagnosis.
