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Laboratory 11 reported 10reference method test portions (including 5 un-inoculated control test
1
portions) that produced non-
Listeria monocytogenes
profiles, with 5 of the test portions
2
producing doubtful profiles of
L. grayi
. Colonies on these plates contained one or more of the
3
following biochemical reactions not typically associated with
L. monocytogenes
: Gram-
4
negative,non-beta-hemolytic and catalase negative.Based on the preliminary biochemical tests
5
conducted the test portions should not have been carried through for final biochemical
6
identification on API
Listeria
strips which resulted in the misidentification of the test portion as
7
Listeria
spp. The selective agar plates for these test portions were sent to the coordinating
8
laboratory for further examination. The coordinating laboratory confirmed the supplementary
9
results (Gram stain, hemolysis and catalase reaction)reported by the participating laboratory and
10
were not able to identify any
Listeria
species. The results from this laboratory were excluded
11
from statistical analysis. The MPN levels obtained for this test portion, with 95% confidence
12
intervals, were 0.59 CFU/test portion (0.46, 0.74) for the low level and 5.41 CFU/test portion
13
(3.53, 8.30) for the high level.For VIDAS LPT test portions, no differences were observed
14
between confirmation of samples using the proprietary chromogenic ALOA and the reference
15
method agar.
16
For the high level, 144 out of 144 test portions were reported as positive by the VIDAS LPT
17
method with all test portions confirming positive.For the low level,70 out of 144 test portions
18
were reported as positive by the VIDAS LPT method with all 70 test portions confirming
19
positive. For the un-inoculated controls, 0 out of 144 samples produced a presumptive positive
20
result by the VIDAS LPT method and no samples confirmingpositive. For test portions analyzed
21
by the AOAC OMA 993.12 method, 144 out of 144 high inoculum and69 out of 144 low
22
inoculum test portions confirmed positive. For the un-inoculated controls, 0 out of 144 test
23
portions confirmed positive.
24
For the low level inoculum, dLPOD
C
values of 0.01 (-0.10, 0.13) wereobtained between the
25
AOAC OMA 993.12 method and the VIDAS LPT method. The confidence intervals obtained for
26
dLPOD
C
indicated no significant difference between the two methods. dLPOD
CP
values of 0.00
27
(-0.12, 0.12) were obtainedbetween presumptive and confirmed VIDAS LPT results. The
28
confidence intervals obtained for dLPOD
CP
indicated no significant difference between the
29
presumptive and confirmed results using either confirmation process.
30
For the high level inoculum, dLPOD
C
values of 0.00 (-0.03, 0.03) were obtained between the
31
AOAC OMA 993.12 method and the VIDAS LPT method. The confidence intervals obtained for
32
dLPOD
C
indicated no significant difference between the two methods. dLPOD
CP
values of 0.00
33
(-0.03, 0.03) were obtained between presumptive and confirmed VIDAS LPT results. The
34
confidence intervals obtained for dLPOD
CP
indicated no significant difference between the
35
presumptive and confirmedresults. Detailed results of the POD statistical analysis are presented
36
in Table 2013.1B below, and Tables 2013.2C-2Dand Figures 2A-2D of Appendix 1.
37
38
ALOA Chromogenic Agar
39
Confirmatory results obtained from the ALOA chromogenic agar were identical to the results
40
obtained from the OXA agar specified by the AOAC OMA 993.12 method. Out of a total of 451
41
positives detected by the VIDAS LPT, 448 were confirmed positive using OXA or ALOA
42
selective agars.
43
44
45
Discussion
46
47
No negative feedback was reported to the study directors from the collaborating laboratories in
48
regards to the performance of the VIDAS LPT assay or the ALOA chromogenic agar. Many
49
11