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VIDAS LPT - Summary 2012
EUROFINS IPL Nord SAS
–
1 rue du Professeur Calmette
–
59046 LILLE cedex
p 7/14
If the number of discrepant results is greater than 22, it is necessary to use the McNemar test with
²
distribution
for one degree of freedom. It consists of determining d = PD
–
ND and comparing d to a minimum value of d
defined for each number of discrepant results.
Number of discrepant results
d minus
d
Conclusion
28
11
13
–
15 = 2
Equivalency
The VIDAS LPT method can be considered to be equivalent to the ISO 11290-1 reference method (including
amendment A1) for the detection of
Listeria
in human food products and environmental samples.
2.1.5 Comments on confirmations
All the characteristic
Listeria
colonies detected after the VIDAS LPT test were confirmed onto either the two
selective agars (Palcam and ChromID OAA), except for one sample (W26) staying negative (isolation after storage
of the LPT broth for 72 hours at 2-8°C).
Regarding the 142 positive consistent results, differences in identification were observed between the reference
method and the VIDAS LPT with 7 products (H7, D14, F8, G11, M2, W9 and H4).
2.1.6 Comments on results obtained after storage at 2-8°C
The results obtained after storing the enrichment broths (positive LPT broths) at 2-8°C for 72 hours are identical to
those obtained directly after 26h incubation.
Four samples (B5, C13, D13 and W26) with false negative results have become positive consistent after broth
storage for 72 hours at 2-8 ° C.
2.2 Relative detection level
The objective was to determine the level of contamination to obtain about 50% of positive results and 50%
negative results.
Various "food matrix-strain" pairs were studied in parallel with the reference method and the VIDAS LPT method,
for the studied categories.
The levels of detection, calculated using the Spearman-Kärber method (LOD
50
)
(1)
, obtained for each "matrix -
strain" combination are :
Relative detection level (CFU / 25 g or 25 mL)
with confidence interval
(2)
LOD
50
Matrix
Strain
Reference method
Alternative method
Rillettes
L. monocytogenes
0,5 [0,3
–
0,9]
0,5 [0,3
–
1,1]
Raw milk
L. ivanovii
0,6 [0,4
–
0,9]
0,5 [0,3
–
1,0]
Red cabbage
L. welshimeri
0,7 [0,3
–
1,5]
0,5 [0,2
–
1,0]
Smoked salmon
L. monocytogenes1/2a
0,7 [0,4
–
1,2]
0,4 [0,2
–
0,8]
Surface samples
L. innocua
0,5 [0,3
–
1,0]
0,6 [0,4
–
1,1]
(1)
”Hitchins A. Proposed Use of a 50% Limit of Detection Value in Defining Uncertainty Limits in the Validation of Presence-Absence
Microbial Detection Methods, Draft 10
th
December, 2003.”
(2)
LOD
50
: estimated level of contamination enabling a positive detection using the alternative method in 50 % of cases.
Conclusion
The level of detection obtained for the alternative method was between 0.3 and 1.1 cells per 25 g or 25 ml. That of
the reference method was between 0.3 and 1.5 cells per 25 g or 25 ml. These results are equivalent.
2.3 Inclusivity and exclusivity
The inclusivity and exclusivity of the alternative method were defined by analyzing, respectively, 50 positive strains
and 30 negative strains