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VITEK® 2 AST - ST Cefotaxime is designed for antimicrobial susceptibility testing of Streptococcus species. VITEK® 2 AST - ST Cefotaxime is a quantitative test 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. Cefotaxime has been shown to be active against most strains of the microorganism listed below, according to the FDA label for this antimicrobial.

Active in vitro and in clinical infections: Streptococcus pneumoniae. Streptococcus pyogenes (Group A betahemolytic streptococci), Streptococcus spp.

The VITEK® 2 Antimicrobial Susceptibility Test (AST) is intended to be used with the VITEK® 2 and VITEK 2 Compact Systems for the automated quantitative or qualitative susceptibility testing of isolated colonies for the most clinically significant aerobic gram-negative bacilli. Staphylococcus spp., Enterococcus spp., Streptococcus agalactiae, and S. pneumoniae.

The VITEK® 2 AST card is essentially a miniaturized, abbreviated and automated version of the doubling dilution technique for determining the minimum inhibitory concentration (MIC). 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.

The VITEK® 2 AST - ST Cefotaxime for Streptococcus species has the following concentrations in the card: 0.25, 0.5, 1, and 2 ug/ml (equivalent standard method concentration by efficacy in uq/ml). The MIC result range for the VITEK 2 card is ≤ 0.125 ~ ≥ 8 µg/ml.

The provided document describes the VITEK® 2 AST - ST Cefotaxime, an antimicrobial susceptibility test system for Streptococcus species. Below is a summary of the acceptance criteria and the study that proves the device meets these criteria.

1. Table of Acceptance Criteria and Reported Device Performance

The device's performance is assessed through Essential Agreement (EA), Evaluable Essential Agreement (Eval EA), Category Agreement (CA), and the rates of major (maj), very major (vmj), and minor (min) discrepancies, against a reference broth microdilution method. While explicit numerical acceptance criteria for each metric are not stated in the provided text, the implied acceptance is based on "high agreement" and low error rates, consistent with guidance documents for AST systems. Typical acceptance criteria for AST systems are often >90% for EA and CA, and low rates for major (<3%), very major (<1.5-2%), and minor errors. The study demonstrates the device meets these expectations.

VITEK® 2 AST - ST Cefotaxime Performance Summary (Combined Clinical and Challenge Data)

Reproducibility (Best Case / Worst Case)

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

• Test Set Sample Size:
  • Clinical Isolates: 1425 isolates for VITEK® 2 AST - ST Cefotaxime with the VITEK® 2 System (1416 viable after removing failed growths).
    • Specifically, 301 clinical isolates of Streptococcus pneumoniae.
  • Stock Isolates: 465 of the 1425 clinical isolates (32.6%) were stock isolates. For S. pneumoniae, 151 of 301 clinical isolates (50.2%) were stock isolates.
  • Challenge Set: 207 isolates (Streptococcus species) and 50 isolates of Streptococcus pneumoniae.
• Data Provenance: Retrospective (stock isolates) and Prospective (recently recovered clinical specimens). The study was conducted at four external clinical sites, but specific country of origin is not mentioned.

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

The ground truth was established using the CLSI broth microdilution reference method. This is an established laboratory standard method, and its execution implicitly relies on qualified laboratory personnel, though no specific number or qualification of "experts" is explicitly stated beyond adherence to the CLSI standard.

4. Adjudication Method for the Test Set

Not applicable. The ground truth was established by comparison to a single, accepted reference method (CLSI broth microdilution). There was no panel of experts adjudicating discrepancies; performance was directly compared to the reference.

5. If a Multi-Reader Multi-Case (MRMC) Comparative Effectiveness Study Was Done

No, this was not an MRMC comparative effectiveness study in the context of human reader performance. This study evaluated an automated antimicrobial susceptibility test system against a reference laboratory method, not human interpretation.

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

Yes, the performance study directly assessed the VITEK® 2 AST - ST Cefotaxime system (algorithm only) against the CLSI broth microdilution reference method. The VITEK® 2 system is an automated device designed to determine MIC values and interpretive categories autonomously once inoculated and loaded.

7. The Type of Ground Truth Used

The ground truth was established using the CLSI broth microdilution reference method, which is an established laboratory standard for antimicrobial susceptibility testing.

8. The Sample Size for the Training Set

The document does not explicitly state the sample size for a "training set." This type of device (an AST system) typically relies on extensive historical data and established microbiological principles for its underlying algorithms, rather than a distinct, separate "training set" in the machine learning sense often discussed for image analysis. The "reading algorithm" for Cefotaxime is unique to that antibiotic, suggesting it's tuned based on known growth kinetics and concentration-response relationships rather than a discrete training set like a diagnostic AI.

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

Given that a specific "training set" is not detailed, the ground truth for establishing the algorithms would likely be based on:

• Extensive historical microbiological data on bacterial growth, antibiotic effects, and MIC definitions.
• Correlation with established reference methods (like broth microdilution) during the development and validation of the VITEK® 2 system's underlying technology and specific antibiotic panels.
• CLSI and FDA interpretive criteria, which define the susceptible, intermediate, and resistant categories based on clinical and microbiological data.

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