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VITEK® 2 AST-Gram Positive Fosfomycin is designed for antimicrobial susceptibility testing of Gram positive microorganisms and 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. VITEK® 2 AST-Gram Positive Fosfomycin is a quantitative test. Fosfomycin has been shown to be active against most strains of the microorganisms listed below, according to the FDA label for this antimicrobial.

Active in vitro and in clinical infections:
Enterococcus faecalis

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 clinically significant aerobic Gram-positive microorganisms to antimicrobial agents when used as instructed.

The principle of the VITEK® 2 AST cards is based on the microdilution minimum inhibitory concentration (MIC) technique reported by MacLowry and Marsh(1) and Gerlach(0). 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 that contains the MIC value along with the interpretive category result for each antibiotic contained on the card.

VITEK® 2 AST-GP Fosfomycin (<8 - ≥256 µg/mL) has the following concentrations in the card: 8, 16, 32 and 128 ug/mL (equivalent standard method concentration by efficacy in ug/mL).

The medical device is the VITEK® 2 AST-Gram Positive Fosfomycin (≤8 - ≥256 µg/mL).

Here's the detailed information regarding its acceptance criteria and the study proving it meets those criteria:

1. Table of Acceptance Criteria and Reported Device Performance

The acceptance criteria for antimicrobial susceptibility test (AST) systems are typically defined by the FDA Class II Special Controls Guidance Document. For the VITEK® 2 AST-GP Fosfomycin, the key performance metrics are Essential Agreement (EA) and Category Agreement (CA), along with the rates of Very Major Errors (VME), Major Errors (ME), and Minor Errors (mE).

Note: The document explicitly states "VITEK® 2 AST-GP Fosfomycin demonstrated substantially equivalent performance when compared with the CLSI agar dilution reference method, as defined in the FDA Class II Special Controls Guidance Document: Antimicrobial Susceptibility Test (AST) Systems; Guidance for Industry and FDA (Issued August 28, 2009)." While specific numerical acceptance thresholds are not explicitly listed in this document, the reported performance clearly meets the general expectations for substantial equivalence based on FDA guidance for AST systems.

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

• Sample Size for Test Set: 399 isolates.
• Data Provenance: The study involved an "external evaluation" conducted with "fresh and stock clinical isolates, as well as a set of challenge strains." This suggests a combination of prospective clinical samples (fresh isolates) and retrospective/archived samples (stock clinical isolates and challenge strains). The country of origin is not specified but is implicitly within the scope of where such FDA-regulated clinical trials are conducted.

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

This type of study (Antimicrobial Susceptibility Testing) does not typically involve human experts establishing a "ground truth" in the way, for example, a radiology AI study would. The ground truth is established by a reference laboratory method. In this case, the CLSI agar dilution reference method was used. This is a standardized laboratory procedure, not reliant on expert subjective interpretation.

4. Adjudication Method for the Test Set

Not applicable. As noted above, the ground truth is established by a standardized laboratory reference method (CLSI agar dilution), not by human expert consensus or adjudication. Discrepancies would be resolved through re-testing or investigation of methods, rather than expert adjudication.

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

Not applicable. This is an antimicrobial susceptibility test system, not an AI-assisted diagnostic imaging system that involves human readers. The VITEK® 2 system is essentially an automated laboratory test.

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

Yes, this study represents a standalone (algorithm only) performance evaluation. The VITEK® 2 system performs the antimicrobial susceptibility testing automatically. Its results are compared directly against the CLSI agar dilution reference method.

7. The Type of Ground Truth Used

The ground truth used was the CLSI agar dilution reference method. This is a well-established, standardized laboratory method for determining the minimum inhibitory concentration (MIC) of an antimicrobial agent against a microorganism, which then translates into interpretive categories (susceptible, intermediate, resistant).

8. The Sample Size for the Training Set

The document does not explicitly state the sample size for the training set. It describes the performance evaluation conducted with "fresh and stock clinical isolates, as well as a set of challenge strains" which refers to the test set. For an AST system like VITEK® 2, the "training" (development and calibration) would involve a substantial, diverse collection of bacterial strains with known resistance profiles, but the specific number used to initially develop the "growth pattern analysis" algorithm is not provided in this summary.

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

The ground truth for the development/training of microbial AST systems, including the VITEK® 2, is typically established using established reference laboratory methods, primarily agar dilution or broth microdilution, in accordance with Clinical and Laboratory Standards Institute (CLSI) guidelines. These methods define the true minimum inhibitory concentration (MIC) for various microorganisms against specific antimicrobial agents. This extensive dataset of MICs across a wide range of strains would be used to develop and fine-tune the "Growth Pattern Analysis" algorithm of the VITEK® 2 system to accurately determine MICs and interpretive categories.

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