Quantification of allergic and irritant patch test reactions using ImageJ

Background/aims It is often difficult to differentiate between allergic and irritant patch test reactions by visual inspection. The purpose of this study was to test an image analysis-based method that differentiates between the two reactions by quantifying the degree of erythema at the patch test site. Methods A total of 172 Japanese patients were patch-tested with sodium lauryl sulfate (SLS) and nickel sulfate, followed by digital photography and visual evaluation of the patch test areas by dermatologists at 48 and 72 h. The digital images were converted to erythema index (EI) images by image processing, and changes in ΔEI (the difference in the EI between the patch test site and the adjacent normal skin) values were analyzed. Results The ΔEI was significantly increased at 72 h relative to that at 48 h for positive nickel sulfate reactions (P < 0.0001), while no significant difference in the ΔEI was found for SLS reactions. Conclusion Using image analysis, allergic patch test reactions may be distinguished from irritant reactions by evaluating the change in the degree of erythema at 48 and 72 h.