Dermoscopy of patch test reactions: study of applicability in differential diagnosis between allergic and irritant reactions

Background: differentiating allergic from irritant patch test reactions may be rather hard, especially when objective features are not distinctive and clinical assessment is scarcely supportive. Objectives: to describe the main dermoscopic features of patch test reactions and to assess the suitability of dermoscopy in differentiating allergic from irritant reactions in clinical setting. Patients/Materials/Methods: consecutive adult outpatients patch tested at our Allergy Unit during a 6-month period who developed any skin reaction were eligible. At the 72-hour reading, allergic and irritant patch test reactions were captured with both a digital camera and a digital dermoscopic system. For each reaction, clinical and dermoscopic variables were separately assessed, scored and then compared. Results: in 94 allergic reactions dermoscopy showed i) intense erythema (100%), ii) dense polymorphic vessels (97.9%), iii) whitish vesicles varying in size, number and arrangement (95.7%), iv) orange-yellowish patchy areas and crusts (35%). Scores of erythema and vessels were significantly higher in allergic than in the 33 irritant reactions, in which vesicles and orange-yellowish patches were a sporadic finding. “Poral pattern” was observed in 85.7% of irritant reactions to cobalt. Conclusions: the dermoscopic patterns of allergic and irritant patch test reactions significantly differ. Highly sensitive and/or specific dermoscopic features may support differential diagnosis.