325 / 1195
Revised Mar 2014
15
For the high level, 131 out of 132 test portions were reported as presumptive positive by the 3M
1
MDA
Salmonella
method with all test portions confirming positive. For the low level, 65 out of
2
132 test portions were reported as presumptive positive by the 3M
™
MDA
Salmonella
method
3
with all test portions confirming positive. For the un-inoculated controls, 1 out of 132 samples
4
produced a presumptive positive result by the 3M MDA
Salmonella
method with all test portions
5
confirming negative. For test portions analyzed by the FDA/BAM Method, 132 out of 132 high
6
inoculum and 70 out of 132 low inoculum test portions confirmed positive. For the un-
7
inoculated controls, 0 out of 132 test portions confirmed positive.
8
For the low level inoculum, a dLPOD
C
value of -0.04 with 95% confidence intervals of
9
(-0.16, +0.09) were obtained between the 3M MDA
Salmonella
method and the FDA/BAM
10
method. The confidence intervals obtained for dLPOD
C
indicated no significant difference
11
between the two methods. A dLPOD
CP
value of 0.00 with 95% confidence intervals of
12
(-0.13, +0.13) were obtained between presumptive and confirmed 3M MDA
Salmonella
results.
13
The confidence intervals obtained for dLPOD
CP
indicated no significant difference between the
14
presumptive and confirmed results using either confirmation process.
15
For the high level inoculum, a dLPOD
C
value of -0.01 with 95% confidence intervals of
16
(-0.04, +0.02) were obtained between the 3M MDA
Salmonella
method and the FDA/BAM
17
method. The confidence intervals obtained for dLPOD
C
indicated no significant difference
18
between the two methods. A dLPOD
CP
value of 0.00 with 95% confidence intervals of
19
(-0.03, +0.03) were obtained between presumptive and confirmed 3M MDA
Salmonella
results.
20
The confidence intervals obtained for dLPOD
CP
indicated no significant difference between the
21
presumptive and confirmed results. Detailed results of the POD statistical analysis are presented
22
in Table 2013.2B and Figures 2A-2B of the Appendix.
23
24
Discussion
25
26
27
For this collaborative study, samples were analyzed at both 25 g and 375 g test portions as
28
required by the current AOAC Guidelines[5], which require methods with more than one sample
29
preparation or enrichment scheme to analyze one matrix per
procedure.Nonegative feedback
30
was provided by the collaborating laboratories in regards to the performance of the candidate
31
method. Several collaborating laboratories expressed questions in regards to the AOAC study
32
design of the collaborative study while other laboratories expressed concern with analyzing 375g
33
test portions. The concern with handling the larger test portions may have contributed to errors
34
observed during testing that resulted in data not used in the statistical analysis.
35
During testing, 4 different laboratories detected the presence of
Salmonella
spp. in 7 raw ground
36
beef uninoculated control test portions. Additionally, 4 different laboratories detected the
37
presence of
Salmonella
spp. in 15 wet pet food uninoculated control test portions. Due to
38
detecting positive samples in the control test portions, the data provided by these laboratories
39
was not included during the statistical analysis. A root cause investigation to determine the
40
source of contamination yielded the following possibilities: Due to the high number of samples
41
analyzed, including test portions inoculated at a high inoculum level, contamination may have
42
occurred during the transfer of enriched samples into the secondary selective enrichments, or
43
during the streaking of the reference agar plates. For the wet pet food, based on feedback from
44
the collaborators, issues with storage during the incubation of the larger test portion sizes may
45
have led to cross contamination of the primary enrichments. Based on the fact that uninoculated
46
control test portions were packaged one day prior to the inoculated test portions, contamination
47