Few studies have reported on the accuracy of reflectance confocal laser microscopy (RCLM) in observing allergic contact dermatitis in vivo. However, distinction of skin reactions from different reagents is not well understood. We sought to diagnose allergic contact dermatitis by RCLM images and compare with routine, visual patch test (PT) reading for 2 major allergen metals in Japan. The PT was performed on the upper back skin with 5% nickel sulfate (Ni) and 2% cobalt chloride (Co) in eight healthy volunteers and eleven patients. RCLM was used to calculate the thickness of the suprabasal epidermis after visual assessment of PT. Comparison of clinical scoring versus suprabasal epidermal thickness was observed. RCLM images of positive PT showed increased suprabasal epidermal thickness on day 2 (D2), and D3 for Co, whereas there was vesicle formation and an overall increase suprabasal epidermal thickness for Ni. In two of 3 doubtful positive PT to Co, RCLM images presented characteristics of irritant reactions; and one characteristic of a positive reaction. The frequency of Co doubtful-positive PT was higher than that of Ni. We found advantages in using RCLM for visualizing features of allergic contact dermatitis and found it a useful tool as an objective parameter in grading severity and types of PT reaction.
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