Validation of a Test System to Detect Low Levels of Listeria moncytogenes in Composite Environmental Sponges and 125G Ready-to-Eat Meat Samples

Introduction:  The USDA’s Food Safety Inspection Service (FSIS) provides the option of combining five environmental sponges for analysis of Listeria monocytogenes. In 2012, the agency also announced a change to the current procedures for product sampling and now allows the compositing of five 25-g samples; increasing the analytical sample size from 25 g to 125 g. Therefore, it is appropriate to ensure the RapidChek  Listeria test system is validated with composite samples.

Purpose: The aim of the study is to validate the RapidChek  Listeria test system for detection of low levels of Listeria monocytogenes in composite sponges and ready-to-eat meat samples

Methods: A method comparison study was conducted to determine accuracy, specificity, and sensitivity of the test system as compared to the USDA reference method (MLG 8.08).  Fifty (50) samples of composite sponges and ready-to-eat meat samples were analyzed by both methods (n = 20 spiked samples plus 5 non-spiked negative controls).   

Results: The RapidChek system reported eighteen (18) confirmed positive results, while the reference method reported ten (10) confirmed positive results in the composite sponge study.  In the composite ready-to-eat meat study, the RapidChek system reported seventeen (17) confirmed positive results while the reference method reported nineteen (19) confirmed positive results.  The relative sensitivity and relative specificity of the test method was 100%.  The overall accuracy of the test method was 121%.  According to Mantel-Haenszel Chi-Square analysis (X²= 2.78), there was no statistical significance between test and reference method.     

Significance: Sample size does not adversely affect the performance of the test method. The target pathogen can be detected at very low levels of contamination in composite sponge samples and 125 g ready-to-eat meat samples.  The study demonstrates the flexibility and robustness of the test method for use by food producers.