Noninvasive in vivo confocal laser scanning microscopy is effective in differentiating allergic from nonallergic equivocal patch test reactions.

Patch testing is the gold standard for the validation of contact dermatitis. It relies on the subjective scoring by an evaluator of the inflammatory reaction induced by an allergen applied to the skin. Equivocal reactions imply faint erythema and could represent allergic, irritant, or negative reactions. They constitute approximately 1 % of the positive reactions encountered in patch test practice. Histological evaluation of the equivocal reaction has proven helpful for the correct interpretation but is however time consuming, and its invasive nature is often unacceptable to the patient. In vivo confocal laser scanning microscopy (CLSM) is a novel, noninvasive imaging technique which permits real-time visualization of skin structures and lesions at a resolution close to that obtained by conventional histology. CLSM has been successfully applied for the differentiation between clinically clear-cut allergic and irritant patch test reactions. The objective of this study is to determine the relevance of CLSM in differentiating between allergic, irritant, and negative equivocal patch test reactions. Fifteen patients who underwent patch testing in our clinic were observed as having 20 equivocal reactions. All 20 reactions were evaluated using in vivo CLSM and compared with adjacent normal skin. In vivo CLSM evaluation revealed that 8 of the 20 equivocal reactions (40 %) showed confocal patterns consistent with the patterns encountered in positive allergic reactions. Anamnestic exposure, i.e., detailed assessment of previous related contact with these allergens, confirmed high relevance rates. In vivo CLSM is useful in differentiating between allergic, irritant, and negative equivocal patch test reactions, a differentiation that cannot be made by conventional clinical patch test reading.