Quantitative evaluation of patch test reactions: a comparison between visual grading and erythema index image analysis

Background/aims: The interpretation of patch test reactions may vary between examiners. As test results are graded, an issue also arises when differing degrees of erythema are placed in the same grade. The purpose of this study was to quantitatively evaluate the degree of erythema in patch tests using image analysis and to study the usefulness of this method by comparing it with visual grading. Methods: A total of 121 Japanese patients were patch tested with various materials. At 48 h, digital photographs of the patch test areas were taken, in addition to a visual evaluation by dermatologists. Digital images of the areas were converted to erythema index (EI) images using image processing and both EI and ΔEI (the difference between the patch test site and adjacent normal skin) values of the patch test sites were compared with the corresponding visual grades. Results: An excellent linear correlation (r=0.95) was found between ΔEI and visual grades, although EI also significantly correlated with visual grades. There were significant differences (P<0.0001–0.05) between the mean ΔEI values of any two adjacent visual grades. Conclusion: ΔEI values derived from image processing appear to be suitable for the quantitative evaluation of erythema in patch tests. This method may be helpful in overcoming the subjectiveness of visual evaluation and for training non‐experts in patch testing.