Determination of the Relative Level of Detection of a Qualitative Microbiological Measurement Method with Respect to a Reference Measurement Method

Performance of qualitative microbiological measurement methods where the results are either "O" (microorganism not detected) or "1" (microorganism detected) is described by their probability of detection (POD) function, i.e., the POD as a function of the level of contamination of the sample, expressed as CFU/g or CFU/mL, or by the level of detection (LODp), i.e., the contamination of the sample that is detected (measurement result "1") with a specified probability p. When it is impossible to obtain samples of known contamination, estimation of the POD and LOD is impossible. However, it may not be the LOD of the method that is of interest, but its LOD with respect to the LOD of a reference method. Hence, an intralaboratory experiment is performed with a reference method, R, and an alternative method, A, at different levels of unknown contamination. A complementary loglog model is used to statistically estimate the relative LOD (RLOD) of A with respect to R that is equal for all chosen values p of the POD. An intralaboratory experiment for the detection of Listeria monocytogenes in fish and eggs illustrates the method. In a simulation study, the bias of the estimate of the RLOD was investigated. This bias is due to the small number of repeated measurements in intralaboratory studies; the relative bias increases with increasing true values of the RLOD from 0 for true RLOD = 1 to about 20% for true RLOD = 3. If the number of CFUs in the test portions does not follow a Poisson distribution, but instead follows a negative binomial distribution, e.g., because of overdispersion, the bias of the estimate of the RLOD decreases. An EXCEL program RLOD_ver1. xlsm for this method of statistical analysis can be downloaded from http://www.wiwiss.fu-berlin.de/ instituteliso/mitarbeiterlwilrichlindex.html.