Purpose: An intralaboratory study evaluating four aerobic count methods and five coliform count methods was conducted to outline an example approach for method selection. Statistical analyses were used to analyze data from each method and a number of testing characteristics (cost, time, incubation temperature, etc.) were used to compare testing methods.
Methods: A cocktail of typical milk isolates was used to inoculate various milk products. Testing data for aerobic counts in ten blind replicate samples and coliform counts in five blind replicate samples were collected by trained, experienced laboratory analysts. Statistical analyses included computation of robust mean, standard deviation, bias, and Mandel’s h and k statistics.
Results: Total aerobic counts across four methods had mean biases ranging from -0.17-0.14 log CFU/ml, while mean bias for coliform count data from five methods ranged from -0.32-0.21 log CFU/ml. Reproducibility standard deviations ranged from 0.04-0.14 log CFU/ml for aerobic counts and 0.05-0.26 log CFU/ml for coliform counts. Cost, preparation time, incubation time and temperature, and limit of quantification were summarized for all methods used. There was no one method that was ideal across all testing characteristics for either analyte.
Significance: This study outlines an example of a practical approach to conduct a method selection study within a single laboratory. The steps taken in this study demonstrate that methods, even when validated, have unique considerations and drawbacks. The process utilized in this study summarizes one way for laboratories to make decisions regarding method selection.