The association between size of test chamber and patch test reaction: a statistical reanalysis

A recent study by Brasch and co-workers reported on the association between size of test chamber and patch test reaction. The investigators interpreted their data on 495 patients as having conclusively shown that standard preparations of fragrance mix, wool wax alcohols, Kathon CG and formaldehyde led to more positive test reactions when large Finn Chambers were used for patch testing. We have scrutinized the statistical aspects of this study and conclude that the authors should have adopted a statistical approach suitable to analyse dependent samples. After explaining the correct methodological way of dealing with quadratic contingency tables formed by 2 dependent samples, we reanalyze the data accordingly and compare the results to those of the original paper. Based on this reanalysis, the conclusions are more complex: the reaction pattern for the fragrance mix and wool wax alcohols is significantly different between small and large test chambers; however, this discrepancy arises primarily from changing weak positive reactions with small chambers to strong positive reactions with large chambers. For formaldehyde, no relationship between chamber size and patch test reaction was found in the data, while for Kathon CG, statistical evidence is borderline that more positive test reactions are yielded by large test chambers than by small ones.