476 / 596
4
following Monday when analysis was initiatedafter a total equilibration time of 96 hours.All
1
samples were packed with cold packs to target a temperature of < 7°C during shipment.
2
In addition to each of the test portions and a separate APCsample, collaborators received a test
3
portion for each matrix labeled as ‘temperature control’. Participants were instructed to obtain
4
the temperature of this portion upon receipt of the package, document the results on the Sample
5
Receipt Confirmation form provided and fax or email it back to the study director.The shipment
6
and hold timesof the inoculated test material had been verified as a quality control measure prior
7
to study initiation.
8
9
Test Portion Analysis
10
11
Collaborators were instructed to follow the appropriate preparation and analysis as outlined in
12
the study protocol for each matrix for both the 3M MDA 2 -
Listeria monocytogenes
method and
13
reference method. For both matrices, each collaborator received 72 test portions (12 high, 12 low
14
and 12 un-inoculated controls for each method to be performed). For the analysis of the deli
15
turkey test portions by the 3M MDA 2 -
Listeria monocytogenes
method, a 125 g portion was
16
enriched with 975 mL of Demi-Fraser (DF) with ferric ammonium citrate (FAC) broth,
17
homogenized for 2 minutes and incubated for 24-
2830
hours at 37 ±1
o
C. For the raw chicken
18
breast fillettest portions analyzed by the 3M MDA 2 -
Listeria monocytogenes
method, a 25 g
19
portion was enriched with 475 mL of DF, homogenized for 2 minutes and incubated for 28-32
20
hours at 37 ±1
o
C.
21
Following enrichment, samples were assayed by the 3M MDA 2 -
Listeria monocytogenes
22
method and, regardless of presumptive result, confirmed following the USDA/FSIS MLG 8.09
23
reference method. Both matrices evaluated by the 3M MDA 2 -
Listeria monocytogenes
method
24
were compared to samples analyzed using the USDA/FSIS MLG 8.09 reference method in an
25
unpaired study design. All positive test portions were biochemically confirmed by the API
26
Listeria monocytogenes
biochemical test or by the VITEK 2 GPbiochemical identification test,
27
AOAC Official Method 2012.02 [8].
28
29
Statistical Analysis
30
31
Each collaborating laboratory recorded results for the reference method and the 3M MDA 2 -
32
Listeria monocytogenes
method on the data sheets provided. The data sheets were submitted to
33
the study director at the end of each week of testing for statistical analysis. Data for each matrix
34
was analyzed using the probability of detection (POD)statistical model [9].
POD statistical
35
analysis was conducted using AOAC Binary Data Interlaboratory Study Workbook, Version 2.3
36
[10].
The probability of detection (POD) was calculated as the number of positive outcomes
37
divided by the total number of trials. The POD was calculated for the candidate presumptive
38
results, POD
CP,
the candidate confirmatory results (
excluding those with presumptive negative
39
resultsincluding false negative results
), POD
CC
, the difference in the candidate presumptive and
40
confirmatory results, dLPOD
CP,
presumptive candidate results that confirmed positive
((including
41
those with presumptive negative results)excluding false negative results)
, POD
C,
the reference
42
method, POD
R
, and the difference in the confirmed candidate and reference methods, dLPOD
C
.
43
A dLPOD
C
confidence interval not containing the point zero would indicate a statistically
44
significant difference between the 3M MDA 2 -
Listeria monocytogenes
and the reference
45
methods at the 5 % probability level. In addition to POD, the repeatability standard deviation
46
(s
r
), the among laboratory repeatability standard deviation (s
L
), the reproducibility standard
47
deviation (s
R
)and the P
T
value were calculated. The s
r
provides the
variabilityvariance
of data
48
within one laboratory, the s
L
provides the difference in standard deviation between laboratories
49