Elucidating the influence of the lower and upper microbiological limits: when is a 3-class sampling plan useful to test for pathogens in food?

The potential advantages of using 3-class sampling plans in the management of pathogens in food has been discussed for years. Using a 3-class plan, some lots would be accepted if, among a total number of samples n, the pathogen was detected in no more than a predefined number of samples c (such as c = 1) exceeding a microbiological limit m, and no sample was at a level above a predefined maximal concentration M. We built upon and extended beyond published methodologies and developed a web application to evaluate the performance of 2-class and 3-class sampling plans. We showed that there is little benefit of a 3-class plan over a 2-class plan with equal values for m, n and c under the classical assumption of a lognormal distribution of bacteria concentration in the lot with a standard deviation of 0.8 log cfu/g, for m value set to “absence in 25g” and M set to values >1 cfu/g. In this situation, the lot will almost always be rejected based on the criterion “>c detected samples” and will almost never (or seldom) be rejected based on only the criterion “at least one sample >M cfu/g”. However, the benefit of a 3-class plan is realized when the distribution of pathogens is not a simple lognormal but includes a proportion of samples contaminated beyond the lognormal distribution, at levels >M. When this is the case, the performance of a 3-class plan relative to the 2-class plan with equal values of m, n, and c improves, then becomes essentially equivalent, as the proportion of highly contaminated samples increases. The behavior of 3-class plan may be useful in such situations. While such distributions of pathogens have been observed across lots in the food supply, better characterization of within-lot contamination distribution in food is needed to better evaluate the performance of sampling plans.