Purpose: Conduct an interlaboratory method validation study to compare performance of the NCIMS-approved Petrifilm™ reference method to the alternative, automated MPN, TEMPO® method for total aerobic bacterial counts using new statistical approaches.
Methods: An interlaboratory method validation study was conducted concurrently with the FDA’s annual milk proficiency test (PT) to compare method performance in five milk types: pasteurized, chocolate, cream, 2%, and raw. Eighteen analysts from 9 laboratories analyzed test portions from each sample in triplicate. Statistics including mean bias and matrix standard deviation (corrected for standard error) were calculated.
Results: Sample-specific bias of the alternative method for total aerobic count suggests that there are no large deviations within the population of samples considered. Based on analysis of 648 data points, mean bias of the alternative method across milk samples for total aerobic count was 0.013 (log) and the confidence interval for mean deviation was -0.066 - 0.009 (log). These results indicate that the mean difference between the selected methods is small and not statistically significant. Matrix standard deviation was 0.077 (log), showing there is a low risk for large sample-specific bias based on milk matrix.
Significance: An innovative multi-laboratory method validation study was conducted by integrating the study into the FDA’s milk PT. Comparing the performance of an automated MPN to a reference method for aerobic count demonstrates how PT data and statistical methods including mean bias and matrix standard deviation can be used to compare method performance in a novel way.