"Extra-edge effect": a further problem in patch test diagnosis?

Sir, In vivo diagnostic tests may cause side-effects. The well-known, typical side-effects of patch testing are active sensitization, irritant reactions, scars, alteration of pigmentation, pustular or microbial infection, reaction to plaster or test devices, and “angry back syndrome” (1). Systemic symptoms are not unusual among patients undergoing patch tests; 5% of tested patients complain of rashes, high temperature and flare-up reactions (2). A particular patch test side-effect, the “edge-effect”, is reported for irritant substances. A similar reaction is also described sporadically for different allergens using Finn Chambers on Scanpor; test substances, not evenly dispersed under the Finn Chamber accumulate around the perimeter of the chamber, causing a stronger allergic reaction at the rim (3, 4). Another peculiar “edge-effect” has been observed testing potent corticosteroids: at the 48-h control the eczematous reaction is evident only on the outer edge of the patch test site, while at the later readings, the entire patch test site becomes eczematous. This phenomenon may be explained by the anti-inflammatory effect of the corticosteroid reinforced by the patch test occlusion during the first 48 h (5). In the case presented here, a strange patch test sideeffect was observed, characterized by an allergic skin reaction localized on the external edge of some patch test cells.