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VITEK® 2 AST-GP Cefoxitin Screen is designed to predict mecA-mediated oxacillin resistance in Staphylococcus spp. It is intended for use with the VITEK® 2 and VITEK® 2 Compact Systems as a laboratory aid in the determination of in vitro susceptibility to antimicrobial agents.

The VITEK® 2 Gram-Positive Susceptibility Card is intended for use with the VITEK® 2 Systems in clinical laboratories as an in vitro test to determine the susceptibility of Staphylococcus spp., Enterococcus spp., and Streptococcus agalactiae to antimicrobial agents when used as instructed.

VITEK® 2 AST-GP Cefoxitin Screen is designed to predict mecA-mediated oxacillin resistance in Staphylococcus spp. The cefoxitin screen and oxacillin work in combination to determine the final interpretation reported for oxacillin. The VITEK® 2 AST-GP Cefoxitin Screen is a qualitative test based on the CLSI, "Disk Diffusion Test for Prediction of mecAmediated resistance in Staphylococci." The VITEK® 2 AST-GP Cefoxitin Screen test is intended for use with the VITEK® 2 and VITEK® 2 Compact Systems as a laboratory aid in the determination of in vitro susceptibility to antimicrobial agents.

The VITEK® 2 card is inoculated with a standardized organism suspension, and growth inside the card is optically monitored throughout the incubation cycle. Results are automatically calculated once a predetermined growth threshold is reached and a report is generated that contains the MIC result and the interpretive category result.

The principle of the VITEK® 2 AST cards is based on the microdilution minimum inhibitory concentration (MIC) technique reported by MacLowry and Marsh('') and Gerlach(2). The VITEK® 2 AST card is essentially a miniaturized, abbreviated and automated version of the doubling dilution technique(3).

Each VITEK® 2 AST card contains 64 wells. A control well which only contains microbiological culture media is resident on all cards. The remaining wells contain premeasured portions of a specific antibiotic combined with culture media. The bacterial or yeast isolate to be tested is diluted to a standardized concentration with 0.45 - 0.5% saline before being used to rehydrate the antimicrobial medium within the card. The VITEK® 2 System automatically fills, seals and places the card into the incubator/reader. The VITEK® 2 Compact has a manual filling, sealing and loading operation. The VITEK® 2 Systems monitor the growth of each well in the card over a defined period of time. At the completion of the incubation cycle, a report is generated. For the VITEK 2 Cefoxitin Screen, the report will list either a positive or negative result. The VITEK 2 Cefoxitin Screen and oxacillin work in combination to determine the final oxacillin interpretation based on the CLSI recommendations.

NOTE: Final determination of the oxacillin interpretation is based on forcing rules as managed by the VITEK 2 Systems software. Both the VITEK 2 AST-GP Cefoxitin Screen test and Oxacillin test must finalize before any forcing rules are applied.

VITEK® 2 AST-GP Cefoxitin Screen has the following concentrations in the card: 4 and 5 ug/mL (equivalent standard method concentration by efficacy in ug/mL).

Here's a breakdown of the acceptance criteria and the study proving the device meets them, based on the provided text:

1. Table of Acceptance Criteria and Reported Device Performance

The device is a qualitative test designed to predict mecA-mediated oxacillin resistance in Staphylococcus spp. The performance is measured by Category Agreement (CA) and various error rates (Very Major Error - VME, Major Error - ME, Minor Error - mE).

Note: The document implicitly defines acceptance by presenting the achieved performance as "acceptable performance." Common regulatory expectations for AST devices have been used to infer typical acceptance targets.

2. Sample Size Used for the Test Set and Data Provenance

• Sample Size for Test Set:
  • S. aureus & S. lugdunensis: 532 isolates tested (Denominator for Category Agreement)
  • Other Staphylococcus species: 267 isolates tested (Denominator for Category Agreement)
• Data Provenance: The study involved an "external evaluation" conducted with "fresh and stock clinical isolates, as well as a set of challenge strains." This indicates a mix of clinical samples and potentially curated reference strains. The document does not specify the country of origin of the data. It is a retrospective study since it uses existing clinical isolates and challenge strains.

3. Number of Experts Used to Establish the Ground Truth for the Test Set and Qualifications of Those Experts

The document does not explicitly state the number of experts or their qualifications. However, it indicates that the device performance was compared against the CLSI disk diffusion method ("Performance Standards for Antimicrobial Disk Susceptibility Tests", Approved Standard -13th Edition (January 2018)). This implies that the ground truth was established by adherence to a standardized, expert-approved reference method, rather than through individual expert consensus on specific cases.

4. Adjudication Method for the Test Set

The document does not describe an adjudication method beyond the direct comparison to the CLSI disk diffusion reference method. There is no mention of a human-in-the-loop process requiring adjudication for conflicting interpretations.

5. If a Multi-Reader Multi-Case (MRMC) Comparative Effectiveness Study Was Done, If So, What Was the Effect Size of How Much Human Readers Improve with AI vs. Without AI Assistance

No, an MRMC comparative effectiveness study was not done. This device is an automated antimicrobial susceptibility testing system, not an AI-assisted diagnostic tool that would typically involve human readers.

6. If a Standalone (i.e., algorithm only without human-in-the-loop performance) Was Done

Yes, a standalone performance assessment was done. The VITEK® 2 AST-GP Cefoxitin Screen is an automated system that generates results directly, which are then compared to the CLSI disk diffusion reference method. There is no human interpretation step in the device's output itself for this performance evaluation. The "report will list either a positive or negative result."

7. The Type of Ground Truth Used

The ground truth used was based on the CLSI disk diffusion method ("Performance Standards for Antimicrobial Disk Susceptibility Tests", Approved Standard -13th Edition (January 2018)), which is a recognized reference standard for antimicrobial susceptibility testing.

8. The Sample Size for the Training Set

The document does not specify the sample size for the training set. It only describes the external evaluation using clinical isolates and challenge strains. Since the device uses a "Discriminant Analysis" algorithm, it implies a training phase, but the details are not provided in this summary.

9. How the Ground Truth for the Training Set Was Established

The document does not explicitly describe how the ground truth for the training set was established. However, given that the final performance is benchmarked against the CLSI disk diffusion method, it is highly probable that the training set's ground truth was also established using this or a similar accepted reference method.

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