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Difference of Probabilities of Detection (dPOD)
.—
Difference of probabilities of
170
detection is the difference between any two POD values.
171
Estimate the dPOD
CP
as the difference between the candidate presumptive result
172
POD (POD
CP
) and the candidate confirmed result POD (POD
CC
) values. The 95%
173
confidence interval on the dPOD
CP
was calculated.
174
175
dPOD
CP
= POD
CP
- POD
CC
176
177
Estimate the dPOD
C
as the difference between the candidate method and
178
reference method POD values. The 95% confidence interval on the dPOD
C
was
179
calculated.
180
181
dPOD
C
= POD
C
– POD
R
182
183
If the confidence interval of a dPOD does not contain zero, then the difference is
184
statistically significant at the 5% level.
185
Results
186
187
The food matrices used, ground beef, queso fresco, deli ham and turkey had low
188
bacterial counts (Table 1). All the samples were screened negative for
Listeria
spp.
189
and
Listeria monocytogenes
. All the food matrices were inoculated with different
190
strains of L.
monocytogenes
(Table 1) to get fractional positives at low inoculation
191
level (0.2-2 CFU/test portion). Additionally, 5 samples were inoculated with high
192
levels of inoculums and 5 were not inoculated (Table 1).
193
Ground Beef: The LMX successfully detected 7 positives out of 25 samples which
194
were confirmed by the USDA method without any false negatives or false positives
195
(POD 0.28; CI 0.14-0.48) (Table 2, 3). Similarly, the USDA reference method was
196
able to detect 7 positives of the 25 samples. The test method was equivalent to the
197
USDA method (dPOD=0; CI -0.24-0.24; chi sq. 0) (Table 3) in detecting
L.
198
monocytogenes
in ground beef.
199
Queso Fresco: Queso fresco enrichments had 11 positives out of 25 low
200
inoculated samples. The LMX successfully detected 11 positives out of 25 samples
201
which were confirmed by the AOAC method without any false negatives or false
202
positives (POD 0.44; CI 0.27-0.63) (Table 2, 3). Similarly, the AOAC reference
203
method was able to detect 11 positives of the 25 samples. The test method was
204
equivalent to the AOAC method (dPOD=0; CI -0.26-0.26, chi sq. 0) (Table 3) in
205
detecting
L. monocytogenes
in queso fresco (Mexican soft-cheese).
206
Deli Ham: Deli ham enrichments had 16 positives out of 25 low inoculated
207
samples. The LMX successfully detected 16 positives out of 25 samples which
208
were confirmed by the USDA method without any false negatives or false positives
209
PTM Certification Modification Report