The relationship between the proportion of the population tested annually and the rate of patients with positive patch test reactions.

BACKGROUND Differences exist between the rates of patients with positive patch test reactions in different centres. OBJECTIVES To explore the varying relationship between the percentage of patients with one or more positive reactions (rate of patients with positive reactions) and the proportion of the population tested annually (intensity of testing) by UK centres contributing to a shared data pool. METHODS Patch test data were pooled from 11 UK centres that use the British Society for Cutaneous Allergy (BSCA), formerly known as the British Society for Contact Dermatitis, database. The catchment population size was estimated for each centre, and the rate of patients with positive reactions to allergens in the BSCA baseline series was calculated. RESULTS As the intensity of testing increases, the rate of patients with positive reactions decreases. We found a strong linear correlation in the range of interest between the intensity of patch testing and the proportion of tests returning a positive result (R(2) = 0.598). CONCLUSION Within the range of testing intensities covered by our reporting centres, the data show that as the intensity of testing increases, the rate of patients with positive reactions decreases.