Abstract
The possible interference of neighbouring allergic patch-test reactions is still an open question. In this study, we investigated whether there is a distance-related mutual modification of neighbouring allergic patch-test reactions to nickel sulfate and potassium dichromate. We used a double-blind multicentre study design with randomized attachment of special TRUE Tests with 1, 3 and 7 cm distance between nickel sulfate and potassium dichromate patches. 589 patients with a history of nickel allergy (523 female, 66 male) were tested, with a mean age of 35 years. A log-linear modelling approach was used for statistical assessment of the relation between the distance separating neighbouring patch tests with nickel and dichromate and the reactions to the allergens. Non-reproducibility coefficients were compared by the generalized version of Fisher's exact test for arbitrary 2-dimensional contingency tables. For the left side of the back, virtually no differences (p=0.70) were found in the reaction patterns obtained for the 3 distances separating nickel and dichromate patch tests. On the right side of the back, the number of reactions to dichromate patches with only 1 cm distance from moderate/strong nickel reactions was lower than the number of positive dichromate tests at larger distances from nickel tests (on the border of statistical significance: p= 0.05). Corresponding side-related results were obtained for subgroups of patients with and without a history of atopic dermatitis. The non-reproducibility of reactions to dichromate was not significantly related to the distance between neighbouring tests. Our data argue against a "spillover" effect of strong/moderate nickel reactions, but indicate that such reactions may, under certain conditions, attenuate adjacent reactions to an unrelated allergen. In the case of future verification, this will have implications for the interpretation of patch tests.
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