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VITEK® 2 AST-Gram Negative Polymyxin B is designed for antimicrobial susceptibility testing of Gram Negative bacilli 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 Negative Polymyxin B is a quantitative test. Polymyxin B 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: Pseudomonas aeruginosa

The VITEK® 2 Gram-Negative 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-negative bacilli 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-GN Polymyxin B has the following concentrations in the card: 0.125, 0.5, 2 and 8 ug/mL (equivalent standard method concentration by efficacy in ug/mL).

The acceptance criteria and study proving the device meets them are described below:

Acceptance Criteria and Device Performance

Note: The acceptance criteria are based on the FDA Class II Special Controls Guidance Document: Antimicrobial Susceptibility Test (AST) Systems; Guidance for Industry and FDA (Issued August 28, 2009). The specific percentage thresholds for "acceptable performance" are derived from this guidance, although the exact numerical thresholds are not explicitly stated within the provided text, they are commonly 90% or higher for both essential and category agreement in AST systems.

Study Information

2. Sample size used for the test set and the data provenance:

• Test Set Size: Not explicitly stated as a single number for each component (clinical isolates, stock clinical isolates, challenge strains). The document mentions an "external evaluation was conducted with fresh and stock clinical isolates, as well as a set of challenge strains." The combined total number of isolates for the test set is not provided.
• Data Provenance: Not explicitly stated (e.g., country of origin). The study used "fresh and stock clinical isolates, as well as a set of challenge strains," suggesting a mix of retrospectively collected and specifically curated (challenge) strains.

3. Number of experts used to establish the ground truth for the test set and the qualifications of those experts:

• Not applicable. The ground truth was established using a reference laboratory method, not expert consensus.

4. Adjudication method for the test set:

• Not applicable. The ground truth was established using a reference laboratory method (CLSI broth microdilution), not through expert adjudication of results from the device.

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, a multi-reader multi-case (MRMC) comparative effectiveness study was not done. This device is an automated antimicrobial susceptibility testing system, not an AI-assisted diagnostic device for human readers.

6. If a standalone (i.e. algorithm only without human-in-the-loop performance) was done:

• Yes, a standalone performance study was done. The VITEK® 2 AST-GN Polymyxin B system, an "Automated quantitative antimicrobial susceptibility test," was compared against the CLSI broth microdilution reference method. The device operates automatically to determine susceptibility.

7. The type of ground truth used:

• Expert consensus, pathology, outcomes data, etc.: The ground truth was established using a CLSI broth microdilution reference method, specifically "the CLSI broth microdilution reference method incubated at 16-20 hours."

8. The sample size for the training set:

• Not explicitly stated. The document describes a performance evaluation (test set) but does not provide details on a separate training set used for developing the algorithm. The VITEK® 2 system's underlying "Growth Pattern Analysis" algorithms would have been developed and trained using diverse microbiological data, but specific sample sizes for this training are not detailed in this 510(k) summary.

9. How the ground truth for the training set was established:

• Not explicitly stated. Given the nature of the device (antimicrobial susceptibility testing), it can be inferred that the ground truth for any training would also have been established using a standardized reference method like broth microdilution, similar to how the test set ground truth was established.

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