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VITEK® 2 AST-Gram Positive Linezolid 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 Linezolid is a quantitative test. Linezolid 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 faecium (vancomycin-resistant isolates only) Staphylococcus aureus (including methicillin-resistant isolates) Streptococcus agalactiae

In vitro data are available, but clinical significance is unknown: Enterococcus faecalis (including vancomycin-resistant isolates) Enterococcus faecium (vancomycin-susceptible isolates) Staphylococcus epidermidis (including methicillin-resistant isolates) Staphylococcus haemolyticus

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 S. agalactiae 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 (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 that contains the MIC value along with the interpretive category result for each antibiotic contained on the card.

VITEK® 2 AST-GP Linezolid has the following concentrations in the card: 0.5, 1, and 2 ug/mL (equivalent standard method concentration by efficacy in us/mL).

Here's an analysis of the provided text, focusing on the acceptance criteria and the study that proves the device meets those criteria:

1. Table of Acceptance Criteria and Reported Device Performance

The acceptance criteria for antimicrobial susceptibility test (AST) systems are generally defined by guidance documents such as the "FDA Class II Special Controls Guidance Document: Antimicrobial Susceptibility Test (AST) Systems." While explicit numerical acceptance criteria (e.g., "EA must be > 90%") are not directly stated in the provided text as a separate table, it can be inferred from the "Performance Overview and Conclusion" section and the structure of the performance data presented.

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

• Sample Size:
  • Enterococcus spp.: 403 isolates (for EA and CA analysis).
  • Staphylococcus spp.: 390 isolates (for EA and CA analysis).
  • Streptococcus agalactiae: 64 isolates (for EA and CA analysis).
  • Total isolates for Linezolid across all species shown in the main table: 403 + 390 + 64 = 857 isolates.
  • Staphylococcus haemolyticus (specific note): 35 isolates were used when evaluating performance for this specific species.
• Data Provenance: The study used "fresh and stock clinical isolates, as well as a set of challenge strains." This indicates a mix of retrospective (stock isolates) and potentially prospective (fresh clinical isolates) data. The geographic origin of the data (e.g., country of origin) is not specified in the provided text.

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

This information is not specified in the provided text. The ground truth was established by the "CLSI agar dilution reference method," which is a laboratory standard rather than a consensus by human experts in the typical sense for imaging or clinical diagnosis. Clinical and Laboratory Standards Institute (CLSI) methods are highly standardized.

4. Adjudication Method for the Test Set

This type of information (e.g., 2+1, 3+1 expert adjudication) is typically relevant to diagnostic devices where human interpretation might be subjective. For an automated antimicrobial susceptibility test system comparing its results to a standardized reference laboratory method (CLSI agar dilution), an adjudication method as commonly understood in medical imaging studies is not applicable or described. The comparison is objective against the reference method.

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

An MRMC study is not applicable here. This device (VITEK 2 AST-GP Linezolid) is an automated system for antimicrobial susceptibility testing, not an AI-assisted diagnostic tool for human readers. There is no human-in-the-loop component described for its core function that would necessitate such a study.

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

Yes, a standalone performance evaluation was done. The VITEK® 2 AST-GP Linezolid system, which is described as an "Automated quantitative antimicrobial susceptibility test," was directly compared to the "CLSI agar dilution reference method." The results presented in Table 2 are the direct output of the VITEK® 2 system. This is a standalone performance evaluation of the device.

7. The Type of Ground Truth Used

The ground truth used was the CLSI agar dilution reference method. This is a validated, standardized laboratory method considered the gold standard for determining minimum inhibitory concentrations (MICs) of antimicrobials.

8. The Sample Size for the Training Set

The provided text describes a "Special 510(k) Submission" for a device that is essentially an updated version of a previously cleared device (K032766) with a change in breakpoints for Staphylococcus species. The study described focuses on validating the performance of this updated device against a reference method.

There is no explicit mention of a separate "training set" size in the context of machine learning or algorithm development. The VITEK® 2 system itself uses "Discriminant Analysis" algorithms, suggesting a historical development and training process, but the current document focuses on the validation of the updated breakpoints rather than the de novo development of the algorithm. If "training set" refers to the data used historically to develop the "Discriminant Analysis" algorithm, that information is not provided.

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

Given that no explicit "training set" is described for this specific submission's validation study, the method for establishing ground truth for a training set (if one was used for the initial development of the VITEK® 2's "Discriminant Analysis" algorithm) is not detailed in the provided text. The current document focuses on comparing the device's output to the CLSI agar dilution reference method for validation.

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