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VITEK® 2 AST-Gram Negative Levolloxacin 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 vito susceptibility to antimicrobial agents. VITEK® 2 AST-Gram Negative Levofloxacin is a quantitative test. Levolloxacin 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: Enterchacter cloacae, Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis, Pseudomonas aeruginosa, Serratia marcescens

Active in vitro but clinical significance unknown: Citrobacter freundii, Enterobacter aerogenes, Klebsiella oxytoca, Morganii, Pantoea agglomerans, Proteus vulgaris, Providencia rettgeri, Providencia stuartii

VITEK® 2 AST-Gram Negative Levofloxacin also reports the susceptibility for the following additional organism as listed on the FDA Susceptibility Test Interpretive Criteria website: Salmonella spp.

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 Levofloxacin has the following concentrations in the card: 0.25, 0.5, 2, and 8 (equivalent standard method concentration by efficacy in ug/mL).

Here's a breakdown of the acceptance criteria and the study details for the VITEK® 2 AST-Gram Negative Levofloxacin device, based on the provided FDA 510(k) summary:

1. Acceptance Criteria and Reported Device Performance

The acceptance criteria are implicitly defined by the FDA Class II Special Controls Guidance Document for Antimicrobial Susceptibility Test (AST) Systems. The performance metrics reported are Essential Agreement (EA), Category Agreement (CA), Very Major Errors (VME), Major Errors (ME), and minor Errors (mE). Reproducibility and Quality Control also demonstrated acceptable results, although specific percentage thresholds for these are not provided in this summary.

Here's the table of reported device performance:

Notes on Interpretation from the document:

• VITEK 2 Levofloxacin MIC values for Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa tended to be in exact agreement or at least one doubling dilution higher when compared to the reference agar dilution method.
• VITEK 2 Levofloxacin MIC values for Serratia marcescens tended to be in exact agreement or at least one doubling dilution lower when compared to the reference agar dilution method.

2. Sample Size and Data Provenance

The sample sizes for the test set are embedded within the performance data (e.g., 346 for Enterobacterales, 225 for Pseudomonas aeruginosa, 48 for Salmonella spp.). These numbers represent the total number of isolates tested for each organism group.

The data provenance is described as an "external evaluation" conducted with "fresh and stock clinical isolates, as well as a set of challenge strains." The document does not specify the country of origin, but "external" implies outside of the manufacturer's immediate control. The use of "fresh and stock clinical isolates" indicates a mix of prospective (fresh) and retrospective (stock) data, augmented by specific challenge strains.

3. Number of Experts and Qualifications for Ground Truth

The document does not explicitly state the number of experts used or their specific qualifications (e.g., "radiologist with 10 years of experience"). However, the ground truth is established by the "CLSI agar dilution reference method incubated between 16-20 hours." This method is a standardized laboratory procedure, and implicitly requires trained laboratory professionals to perform and interpret, but it's not based on expert consensus in the typical sense of diagnostic imaging or clinical interpretation.

4. Adjudication Method

The concept of "adjudication" in the sense of multiple experts reviewing and reaching a consensus on a case (like 2+1 or 3+1) is not applicable here. The ground truth is determined by a single, standardized reference laboratory method (CLSI agar dilution). Any discrepancies would be between the VITEK 2 device's result and this reference method, leading to the error rates (VME, ME, mE) reported.

5. Multi-Reader Multi-Case (MRMC) Comparative Effectiveness Study

No, a Multi-Reader Multi-Case (MRMC) comparative effectiveness study was not conducted as described. This type of study typically involves human readers or clinicians making diagnoses with and without AI assistance to measure improvement in human performance. The VITEK 2 device is a fully automated laboratory device, and the study compares its performance directly against a reference laboratory method, not against human interpretation of the images/data it produces.

6. Standalone Performance Study

Yes, a standalone (algorithm only without human-in-the-loop performance) study was done. The entire performance evaluation reported in the 510(k) summary is of the VITEK 2 device's output (MIC values and interpretive categories) compared directly to the CLSI agar dilution reference method. The device is intended to be automated, and its performance is assessed independently.

7. Type of Ground Truth Used

The ground truth used is the CLSI agar dilution reference method results. This is a recognized standard laboratory method for determining antimicrobial susceptibility, considered the gold standard for this type of in vitro diagnostic test. It is not based on expert consensus, pathology, or outcomes data in the usual clinical sense, but rather a robust, objective laboratory measurement.

8. Sample Size for the Training Set

The document does not specify the sample size for the training set. It describes the data used for the "external evaluation" (test set) but provides no information about the isolates used to develop or train the VITEK® 2 AST-GN Levofloxacin system's algorithms (Discriminant Analysis).

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

The document does not provide details on how the ground truth for the training set was established. However, given the nature of AST systems, it is highly probable that the training set's ground truth would have also been established using a similar, if not identical, reference method like the CLSI agar dilution method. The device's "Discriminant Analysis" algorithms would have been developed and optimized to correlate its readings with these reference standard results.

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