Patch test reactions through the lens of dermoscopy: Further insights, particularly on weak allergic reactions.

Background: Distinguishing weak allergic reactions from irritant patch test reactions may be difficult.

Objectives: To describe the dermoscopic features of allergic reactions (especially weak allergic ones) and irritant patch test reactions, and to assess the suitability of dermoscopy in supporting differential diagnosis.

Methods: Eligible participants for this observational, cross-sectional study included consecutive adult outpatients patch tested during a 12-month period, who developed any skin reaction. Healthy volunteers were patch tested with sodium lauryl sulfate as irritant controls. At the 72-hour reading, patch test reactions were recorded both with a digital camera and a digital dermoscopic system. For each reaction, clinical and dermoscopic variables were separately assessed, scored, and then compared.

Results: Erythema, vessels, and vesiculation were constant dermoscopic features of allergic reactions (n = 173). In 46 weak (+) allergic reactions, dermoscopy showed (a) erythema (100%), (b) dense polymorphic vessels (100%), and (c) whitish vesicles (78.3%). Scores for vesicles and dotted vessels were significantly higher in weak allergic than in irritant reactions. Vesicles were identified as the chief dermoscopic parameter for correctly distinguishing weak allergic from irritant reactions.

Conclusions: Dermoscopy can improve accuracy in the differential diagnosis between weak allergic and irritant patch test reactions.