The reaction index and positivity ratio revisited

Assessing the quality of patch test preparations continues to be a challenge. 2 parameters, the reaction index (RI) and positivity ratio (PR), have been proposed as quality indicators by the Information Network of Departments of Dermatology (IVDK). The value of these statistics has not been examined by others.

We calculated the RI and PR for the 24 TRUE test allergens in 7635 patients with consecutive eczema tested at the Department of Dermatology, Odense University Hospital over a 15-year period, and data for 7 selected standard allergens were compared with corresponding data from the original IVDK papers.

At our patch test laboratory, RI's are lower, even negative, and the PR's are considerably lower than the IVDK results. The most likely explanation is a difference in reading/scoring tradition between the different departments of the IVDK and our department. Calculation of RI's and PR's for patch test allergens is of limited value as a measure of quality of the patch test materials, because it predominantly reflects differences in scoring and reading of patch test reactions. Further, questionable reactions (+?) may be clinically relevant and very important for the individual patient. Focus on standardization of patch test materials, patch test technique, and reading of patch test reactions is mandatory.