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 lest chamber and patch lest reaction. The investigators interprete d their data on 495 patients as having conclusively shown that standard pre parations of fragrance mix, wool wax alcohols, Kathon CG and formaldehyde l ed to more positive test reactions when large Finn Chambers were used for p atch testing. We have scrutinized the statistical aspects of this study and conclude that the authors should have adopted a statistical approach suita ble to analyse dependent samples. After explaining the correct methodologic al way of dealing with quadratic contingency tables formed by 2 dependent s amples, we reanalyze the data accordingly and compare the results to those of the original paper. Based on this reanalysis, the conclusions are more c omplex: the reaction pattern for the fragrance mix and wool wax alcohols is significantly different between small and large test chambers; however, th is discrepancy arises primarily from changing weak positive reactions with small chambers to strong positive reactions with large chambers. For formal dehyde, no relationship between chamber size and patch test reaction was fo und in the data, while for Kathon CG, statistical evidence is borderline th at more positive test reactions are yielded by large test chambers than by small ones.