Estimation of the Detection Limit of Fecal Coliforms and <i>Staphylococcus aureus</i> Qualitative Methods

Abstract

ISO/IEC 17025 : 2017 requires laboratories to verify the performance of methods before introducing them. ISO 16140-3 : 2021 employs LOD50 (the level of detection for which 50% of tests give a positive result) as an index for evaluating the performance of microbial qualitative methods. While fecal coliforms and Staphylococcus aureus qualitative methods are widely implemented in Japan, their LOD50 values have not been established, making performance evaluation of these methods impossible at the time of introduction. As a result, we looked into estimating the LOD50 values of fecal coliforms and S. aureus qualitative methods to evaluate the index of method performance. The fecal coliforms method had an LOD50 value of 19–31 CFU/mL and the S. aureus method had an LOD50 value of 29–49 CFU/mL by laboratory personnels using samples without food matrix. We estimated LOD50 of six food samples using each method in a single laboratory. The LOD50 values of all foods calculated using the fecal coliforms method were in the range of 14–27 CFU/g and those calculated using the S. aureus method were in the range of 25–48 CFU/g, indicating that these methods did not have low levels of sensitivity detection compared with skill variation. LOD50 values for food samples with the highest or lowest LOD50 in each method were compared among five laboratories. Because the estimated LOD50 values of both the methods were within four times of each laboratory result, which is the acceptable limit specified in ISO 16140-3 : 2021, they were assumed to be suitable values. This study is expected to be utilized as a benchmark in the verification of method performance in each laboratory.