ABSTRACT
Background: We deal with interlaboratory experiments (collaborative studies) in which k participating laboratories, selected randomly from a population of laboratories, use samples from one and the same material or matrix. They perform binary microbiological measurements for which the measurement results are either "0" (target microorganisms not detected) or "1" (target microorganisms detected). The performance of such a measurement method is described by its probability of detection (POD) function, i.e., the POD as a function of the contamination of the sample (CFU per gram or CFU per milliliter), or by the level of detection (LODp), i.e., the contamination level of the sample that is detected (measurement result "1") with a specified probability p. Objective: We derive an approximate statistical analysis that is simple enough to be implemented in a spreadsheet application. Methods: Under the assumption of a Poisson distribution of the number of CFU in the samples, we estimate the mean POD function of the laboratories and the SD of the laboratory effect based on a complementary log-log model, a special case of the Generalized Linear Model in the special situation in which the contamination level is known by means other than the POD. The estimates are obtained by maximization of the Laplace approximation of the likelihood function. By simulation, a bias correction factor for the estimate of the SD is obtained. With the estimated POD function, LODs can be estimated. The model can also be used to evaluate the relative LOD of an alternative method with repect to a reference method. Results: The EXCEL program PODLOD-interlab_ver1.xls for this method of statistical analysis can be downloaded from
Subject(s)
Microbiological Techniques/statistics & numerical data , Likelihood Functions , Limit of Detection , Poisson DistributionABSTRACT
Recent foodborne crises have demonstrated the importance of monitoring food safety. In terms of microbiological criteria, food safety requires the reliable detection of pathogens such as Listeria monocytogenes along the food chain by appropriate analytical methods. However, indications exist that accompanying Listeria innocua strains suppress the growth of L. monocytogenes during selective enrichment, which may cause reduced or even inhibited detection. To study these effects, the limit of detection of L. monocytogenes was investigated in the presence of L. innocua using the International Organization for Standardization standard method ISO 11290-1 and the VIDAS LDUO system, an automated method based on enzyme-linked fluorescence technology. The challenge was to provide low initial Listeria concentrations at sufficient precision to quantify the influence on the probability of detection of L. monocytogenes. The application of reference materials appropriate for quantitative test methods and a standardized dilution procedure were necessary to ensure accurate CFU levels of defined proportions of mixtures of both Listeria species. During selective enrichment, overgrowth of L. monocytogenes by L. innocua could be confirmed, leading to high rates of false-negative results. Moreover, with both methods, a significant decrease in the detectability of L. monocytogenes could be quantified at ratios of 2:1 at very low concentrations representative of natural contamination levels often found in foods and environments. It is concluded that there is a need to improve existing procedures with respect to selective enrichment, as well as the detection techniques.
Subject(s)
Colony Count, Microbial/methods , Colony Count, Microbial/standards , Culture Media/chemistry , Listeria monocytogenes/isolation & purification , Listeria/isolation & purification , Bacterial Typing Techniques , Consumer Product Safety , Food Microbiology , Listeria/classification , Listeria monocytogenes/classification , Sensitivity and Specificity , Species SpecificityABSTRACT
Qualitative microbiological measurement methods in which the measurement results are either 0 (microorganism not detected) or 1 (microorganism detected) are discussed. The performance of such a measurement method is described by its probability of detection as a function of the contamination (CFU/g or CFU/mL) of the test material, or by the LOD(p), i.e., the contamination that is detected (measurement result 1) with a specified probability p. A complementary log-log model was used to statistically estimate these performance characteristics. An intralaboratory experiment for the detection of Listeria monocytogenes in various food matrixes illustrates the method. The estimate of LOD50% is compared with the Spearman-Kaerber method.
