Abstract

A feasibility study was undertaken for the certification of a reference material containing low numbers of Salmonella. A batch of 8800 capcules was prepared and eleven laboratories each enumerated 50 capsules from this batch, according to a standardized procedure. The results from four laboratories were excluded from the analysis of data after the first step of the statistical analysis based on their results found for homogeneity and mean contamination level. The remaining seven laboratories found contamination levels that did not differ significantly from each other. The mean contamination level found by these seven laboratories was 5.65 cfu/capsule. The homogeneity, based on all counts by these seven laboratories, was not fully conform to a Poisson distribution. A ratio of variance to mean of 1.18 was found, which is considered acceptable. The proportion of negative capsules found in the study was 0.58% and the upper limit of the corresponding confidence interval (one sided, (alpha = 0.05) was 1.8%. Concluded was that the batch prepared is of sufficient to be used for a certification study.