Positive nickel patch tests do not intensify positive reactions to adjacent patch tests with dichromate - Results from a double-blind multicentre study of the German Contact Dermatitis Research Group (Deutsche Kontaktallergie-Gruppe, DKG)

The possible interference of neighbouring allergic patch-test reactions is still an open question. In this study, we investigated whether there is a d istance-related mutual modification of neighbouring allergic patch-test rea ctions to nickel sulfate and potassium dichromate. We used a double-blind m ulticentre study design with randomized attachment of special TRUE Tests wi th 1, 3 and 7 cm distance between nickel sulfate and potassium dichromate p atches. 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 se parating neighbouring patch tests with nickel and dichromate and the reacti ons to the allergens. Non-reproducibility coefficients were compared by the generalized version of Fisher's exact test for arbitrary 2-dimensional con tingency 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 sep arating nickel and dichromate patch tests. On the right side of the back, t he number of reactions to dichromate patches with only 1 cm distance from m oderate/strong nickel reactions was lower than the number of positive dichr omate tests at larger distances from nickel tests (on the border of statist ical significance: p= 0.05). Corresponding side-related results were obtain ed for subgroups of patients with and without a history of atopic dermatiti s. 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 tha t such reactions may, under certain conditions, attenuate adjacent reaction s to an unrelated allergen. In the case of future verification, this will h ave implications for the interpretation of patch tests.