Noninvasive Imaging Modalities for Evaluating Patch Testing: A Systematic Review

Despite being the gold standard for diagnosing allergic contact dermatitis (ACD), interpretation of patch test reactions remains subjective, relying heavily on visual assessment and palpation. This presents challenges when distinguishing between doubtful, weak allergic, and irritant reactions, particularly in patients with skin of color, where erythema may be less apparent. Recent updates to the International Contact Dermatitis Research Group (ICDRG) classification system underscore the need to refine criteria for more consistent interpretation and to improve assessment methods across diverse skin types. Noninvasive imaging technologies offer promising alternatives to traditional visual grading by enabling objective characterization of patch test reactions. In this systematic review, we assess 38 studies investigating various modalities, including ultrasound, thermography, optical coherence tomography, reflectance confocal microscopy, and 3D imaging systems. Each modality was evaluated for its mechanism, measurable variables, alignment with ICDRG criteria, ease of use, and ability to differentiate patch test reaction grades. While several tools demonstrated potential in identifying positive reactions, many were limited to binary classification and lacked validation in diverse populations. Integration into clinical practice remains limited by cost, training, and variability in data interpretation. Further research incorporating the updated ICDRG guidelines and broader representation of skin tones is essential to determine the utility of these modalities. Improving objectivity in patch test evaluation may ultimately lead to more equitable and accurate diagnosis of ACD.