The “edge effect” with patch test materials

Abstract

A positive "edge effect", i.e., the accumulation on the skin of a chemical solution (such as fluorescein 0.01% in a 50/50 water-ethanol solution) at the periphery of the patch test sites has been demonstrated. It occurs with different test materials (Finn Chamber; Silver Patch Test; Patch Test Chamber). Practical implications are discussed: this observation could be important when discussing results of laboratory investigations. In clinical practice, it could explain the occurrence of "ring-shaped" positive allergic patch test reactions to chemicals used in solution, i.e., Kathon CG or hydrocortisone.

Reactivity at Edges of Corticosteroid Patch Tests May Be an Indicator of a Strong Positive Test Response

“…A concentration of allergen at the margin of patch tests was discussed in 1958 by Sultzberger [10]. Fischer and Maibach [11]and later Fyad et al [12]have described accumulation of dye at the margins of test patches. Aluminum sensitivity may result in ring-shaped reactions owing to allergen release from the flange of aluminum Finn Chambers [13, 14].…”

“…The term has even been used in studies on dye distribution within patch tests [11, 12]. We suggest this term be used for all kinds of phenomena specific to edges of patches such as the pallor observed in this study.…”

“…The mechanisms behind a reactive edge effect are not fully understood [10, 11, 12, 15, 16]. For Finn Chambers, there may be a better penetration of test substances in the epidermis under the flange of the ring owing to its pressure [11].…”

“…For the squares, such an accumulation may be the case in small spaces formed just outside the border of squares, between the tape and the skin. Capillary migration [12, 15]and sweat [11]may even cause an accumulation of test substances at patch edges.…”

Contact allergy to corticosteroids

“…On the contrary, another phenomenon is a reactive vasodilation, presenting as an impalpable erythema on the patch-test site; this reaction certainly has to be kept under observation because it could well be the beginning of an allergic reaction. Unlike the ordinary edge effect (31), which is regarded as irritation occurring under the edge of the patch-test chamber, the``edge effect'' observed with corticosteroids is a rather unique phenomenon: at the ®rst reading, there is often in®ltration and erythema only on and around the edge of the patch-test chamber with a completely clear center. Later, this center also becomes eczematous as well.…”