Purpose: Validation sampling was performed to ensure the Listeria spp. frequency detected during routine sampling did not underestimate the Listeria spp. frequency in a given facility.
Methods: Using six “artisan” US dairy plants’ environmental pathogen monitoring programs, sample size calculations were performed to determine how many environmental samples needed to be collected to have an 80% power to determine whether the Listeria spp. frequency observed during validation sampling was no more than twice the frequency as estimated by the routine sampling. Sample size calculations were performed using the routine sampling results to estimate the minimum detectable difference in the validation sampling using a normal approximation to the binomial distribution. A minimum of 50 samples were collected and tested for Listeria spp. according to the FDA BAM from each plant to ensure a confidence interval within the validation goal.
Results: Prevalence of Listeria spp. varied by plant during both routine sampling (< 0.3% - 22.64%) and validation sampling (1.33% - 14%). Five of the six plants in the study met their validation prevalence goal, suggesting effective environmental pathogen monitoring plans.
Significance: This study provides a model for a science- and statistics-based method of validating environmental pathogen monitoring programs.