AOAC RI ERP Final Action Recommendations eBook

FINAL (Version 4)

DuPont™ BAX ® System Real-Time PCR Assay for Salmonella : Collaborative Study

POD analysis (the 95% confidence interval of the dLPOD included 0 in all cases). Two orange juice 1 samples (one from each of two collaborator sites) returned a presumptive positive result with the test 2 method but could not be culture confirmed. One sample indicated a very weak positive result, 3 suggesting either a cross-contamination event (most likely during a sample transfer step) or a very low 4 target cell density in the sample, which could be detected with the PCR method but was difficult to 5 detect by culture. The second sample returned a strong positive result with the test method, so it is 6 unclear what caused the discordant results between the test and reference methods. The remaining 502 7 orange juice samples tested from the alternative enrichment were in agreement with culture 8 confirmation from the alternative enrichment broths. 9 The results for frankfurters are summarized in Tables 4 – 6. At each inoculation level, the BAX® System 10 method and the reference method demonstrated no significant statistical difference as indicated by 11 POD analysis (the 95% confidence interval of the dLPOD included 0 in all cases). Two frankfurter 12 samples, both from the same collaborator site, returned a presumptive positive result with the test 13 method but could not be culture confirmed. Both samples indicated a very weak positive result, 14 suggesting either a cross-contamination event or a very low target cell density in the sample, which 15 could be detected with the PCR method but was difficult to detect by culture. The remaining 502 16 frankfurter samples analyzed with the alternative method were in agreement with culture confirmation 17 results. One sample initially returned an indeterminate result with the test method and was re-tested 18 according to the manufacturer’s instructions. Upon retest, this sample returned a negative result, which 19 was in agreement with culture confirmation results. 20 A POD summary of all test method results is shown in Table 7. Across all 3 inoculation levels for both 21 matrices, statistical analysis indicates that the test method presented demonstrates no significant 22 differences from the reference methods. The within laboratory component (S r ) of the reproducibility S R 23 value represents the sampling variability at very low spiking levels. It accounted for all of the S R value 24 observed for each matrix collaboratively studied, the S L value (between laboratory effect components of 25 S R ) being zero in both data sets at each partial response spike level. This acceptable inter-laboratory 26 reproducibility is supported by the insignificant homogeneity test P T values (>0.1), which suggest that 27 the laboratory POD values are not significantly different when allowance is made for the sampling 28 variability. While interpretation of this latter test is subject to the study design, 10 or more laboratories 29 with 12 replicate sample portions per level for each of three levels (high, low and unspiked) per 30 laboratory is deemed adequate for such studies. 31 The graphical representation of the data (Fig. 1) demonstrates that the dose response curve for each 32 matrix encompasses the partial response region required for qualitative detection method analysis. The 33 95% confidence interval of each dPOD value determined at each concentration contains zero, which is 34 indicative of no significant difference between the candidate and reference methods and between the 35 candidate presumptive result and candidate confirmed result.

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DuPont Nutrition & Health

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