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Test Portion Analysis
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Collaborators followed the appropriate preparation and analysis protocol according to the
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method for each test portion size. For both test portion sizes, each collaborator received 72 test
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portions of each food product (12 high, 12 low and 12 controlsper method). For the analysis of
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the 25gram test portions by the VIDAS LMXmethod,each samplewasenriched with 225 mL of
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prewarmed (18-25
o
C) LMX broth containing LMX supplement (500µL supplement/225mL
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LMX broth) and homogenized for 2 minutes. Test portions were incubated for 26-30 hours at 37
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±1
o
C. For the 125 gram test potions analyzed by the VIDAS LMXmethod, each sample was
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enriched with 375mL pre-warmed (18-25
o
C) LPT broth andhomogenized for 2 minutes. Test
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portions were incubated for 24-30hours at 30 ±1
o
C. For 125 gram test portions only, a 1.0 mL
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aliquot of the primary enrichment was transferred into 10 mL LPT broth and incubated for 22-26
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hours at 30 ± 1
o
C
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Following enrichment, samples were assayed by the VIDAS LMX method and confirmed
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following procedures outlined in the reference method by streaking an aliquot of the primary
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enrichment onto Oxford Agar (OXA) and a proprietary chromogenic agar, ALOA. Presumptive
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positive samples were streaked for isolation on TSA/ye and biochemically confirmed by
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morphology verification via Gram stain, hemolysis test and by VITEK 2 GP Biochemical
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Identification method (AOAC OMA 2012.02) or API Listeria biochemical test kits. Laboratories
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utilizing API Listeria kits were also required to conduct a catalase test and an oxidase test.
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Both test portion sizes analyzed by the VIDAS LMX methods were compared to samples
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(25 g) analyzed using the AOAC 993.12 reference method in conjunction with VITEK 2 GP
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Biochemical Identification (OMA 2012.02) or API
Listeria
(8) for the confirmation of
Listeria
in
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an unpaired study design. Twenty-five gram test portions were enriched in pre-warmed (45
o
C)
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selective enrichment broth, homogenized for 2 minutes and incubated at 30 ± 2
o
C for 48 hours.
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Samples were streaked onto OXA and presumptive positive samples were streaked for isolation
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onto TSA/ye. Colonies from TSA/ye were confirmedby morphology verification via Gram stain,
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hemolysis test and by VITEK 2 GP Biochemical Identification method or API Listeria
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biochemical test kits. Laboratories utilizing API Listeria kits were also required to conduct a
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catalase test and an oxidase test.
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Statistical Analysis
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Each collaborating laboratory recorded results for the reference method andVIDASLMX
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results. The data sheets were submitted to the study director at the end of each week of testing
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for analysis. The results of each test portion for each sample were compiled by the study director
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and the qualitative VIDAS LMXresults were compared to the reference method for statistical
37
analysis. Data for each test portion size was analyzed using the probability of detection (POD)
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statistical model [5, 6]. A confidence interval of a dLPOD not containing the point zero would
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indicate a statistically significant difference between the VIDAS LMXmethod and the AOAC
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OMA 993.12 reference method at the 5 % probability level [9].
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AOAC Official Method 2013.xxx
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