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BBL™ CHROMagar™ MRSA is a selective and differential medium for the qualitative direct detection of nasal colonization by methicillin resistant Staphylococcus aureus (MRSA) to aid in the prevention and control of MRSA infections in healthcare settings. The test is performed on anterior nares swab specimens from patients and healthcare workers to screen for MRSA colonization. BBL™ CHROMagar™ MRSA is not intended to diagnose MRSA infection nor to guide or monitor treatment for infections.

The BBL CHROMagar MRSA medium permits the direct detection and identification of MRSA through the incorporation of specific chromogenic substrates and cefoxitin. The cephalosporin (Cefoxitin) is incorporated to select for methicillin resistant strains of Staphylococcus aureus. MRSA strains will grow in the presence of cefoxitin and produce mauve-colored colonies resulting from hydrolysis of the chromogenic substrate. Additional selective agents are incorporated for the suppression of gram-negative organisms, yeast and some gram-positive cocci. Bacteria other than MRSA may utilize other chromogenic substrates in the medium resulting in blue to blue/green colored colonies or if none of the chromogenic substrates are utilized, appear as white or colorless.

This document describes the performance of the BBL™ CHROMagar™ MRSA (CMRSA) culture medium for the direct detection of methicillin-resistant Staphylococcus aureus (MRSA) from anterior nares swab specimens.

Table of Acceptance Criteria and Reported Device Performance

The acceptance criteria for the BBL CHROMagar MRSA are not explicitly stated as numerical targets in the provided text, but rather implied by the achievement of sufficient performance for a 510(k) clearance and comparison to predicate devices and other methods. Based on the performance data presented, the implied criteria focused on high sensitivity and specificity. The "Performance" section under Analytical Studies mentions a sensitivity and specificity criteria of ≥90% was met for CMRSA compared to Oxacillin MIC testing and Oxacillin Screen Agar testing (with 48-hour incubation).

Study Details:

1. Sample sizes used for the test set and the data provenance:

   • Clinical Studies (Primary Test Set): 1,974 compliant nares surveillance specimens.
   • Clinical Studies (Sub-analysis for susceptibility comparison): 325 specimens for comparison with Oxacillin MIC and Oxacillin Screen Agar (comprising 117 MRSA and 208 MSSA for MIC, and 116 MRSA and 209 MSSA for Screen Agar).
   • Clinical Studies (Comparison to other methods): Sample sizes for MRSA (118 for Cefoxitin Disk, 123 for PBP2', 116 for PCR) and MSSA (204 for Cefoxitin Disk, 201 for PBP2', 202 for PCR) were used.
   • Analytical Studies (Reproducibility): 17 test strains.
   • Analytical Studies (Expression of Resistance): 17 strains (11 heterogeneous, 5 homogeneous, 1 negative).
   • Clinical Reproducibility Testing: 10 S. aureus (MSSA and MRSA) strains.
   • Challenge Strain Testing: 20 S. aureus (MSSA and MRSA) strains.

   Data Provenance: The clinical studies were conducted at "four geographically diverse clinical sites, composed of regional hospitals and university-based laboratories." This suggests a prospective collection from potentially different regions within the country (likely the US given the FDA submission). Both patient and healthcare worker specimens were included.

2. Number of experts used to establish the ground truth for the test set and the qualifications of those experts:
   The document does not specify the exact number or qualifications of experts (e.g., medical technologists, clinical microbiologists) involved in establishing the ground truth for the clinical specimens or analytical studies. The ground truth methods used were primarily laboratory-based gold standards (Oxacillin MIC, Oxacillin Screen Agar, PBP2' Latex Agglutination, Cefoxitin Disk Diffusion, PCR for mecA gene, and traditional culture on TSA) which inherently involve trained personnel interpreting results. However, there's no mention of a specific expert consensus panel for establishing the "truth" of each clinical specimen in the way typically seen in image-based diagnostic studies.

3. Adjudication method for the test set:
   Not applicable in the direct sense of human reader disagreement for diagnostic interpretation. The methods establishing ground truth (e.g., Oxacillin MIC, PCR) are objective laboratory tests. For the CMRSA itself, the differentiation of MRSA colonies was based initially on "mauve-colored colonies" and then "specificity was improved by confirming mauve colonies from CMRSA with morphology determination and coagulase testing." This suggests a two-step interpretation process, with a confirmatory test.

4. 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:
   Not applicable. This is a study for a culture medium (a laboratory diagnostic device), not an Artificial Intelligence (AI) device or an imaging study involving human readers. Therefore, an MRMC study or assessment of AI assistance is not relevant to this submission.

5. If a standalone (i.e., algorithm only without human-in-the-loop performance) was done:
   This device is a culture medium. Its performance is inherent to the medium itself (i.e., how it differentially grows and colors colonies) and its interpretation is done by trained laboratory personnel. The "standalone" performance is effectively the inherent performance of the medium within the described laboratory workflow. The document describes the raw performance of the medium in detecting MRSA without external human modification of the primary test result. The "colony color alone" results were presented, followed by improved specificity with confirmatory testing, indicating the primary interpretation of the medium.

6. The type of ground truth used (expert consensus, pathology, outcomes data, etc.):
   The ground truth was established using a combination of recognized laboratory gold standard methods for MRSA detection:

   • Oxacillin MIC (Minimum Inhibitory Concentration): A standard method for determining antimicrobial resistance.
   • Oxacillin Screen Agar: A predicate device used for determining Staphylococcus aureus resistance to oxacillin.
   • PBP2' Latex Agglutination: A test that detects the penicillin-binding protein 2a, which mediates methicillin resistance.
   • Cefoxitin Disk Diffusion: A standard antimicrobial susceptibility test using cefoxitin as a surrogate marker for methicillin resistance.
   • PCR detection of the mecA gene: The molecular gold standard for detecting the gene responsible for methicillin resistance in staphylococci.
   • Isolation on TSA II/TSA (Tryptic Soy Agar): Non-selective media used for organism isolation, followed by identification and susceptibility testing.
   • Confirmatory Coagulase Test: A biochemical test to confirm S. aureus species.

7. The sample size for the training set:
   The document does not explicitly delineate a "training set" in the context of machine learning or AI models. Given that this is a culture medium, the development process would involve extensive laboratory strains (likely hundreds or more during research and development phases not detailed in the 510(k) summary) to optimize the chromogenic formulation, selective agents, and concentrations. The "17 test strains" for reproducibility and "17 strains" for expression of resistance in the analytical studies are part of the validation/verification rather than a machine learning training set. The "panel of twenty (20) challenge strains" in clinical studies are also for validation.

8. How the ground truth for the training set was established:
   As there is no explicit "training set" in the sense of a machine learning model, this question is not directly applicable. For the development and optimization of the culture medium, the ground truth for candidate strains would have been established using established microbiological techniques (e.g., biochemical tests, traditional culture, and antimicrobial susceptibility testing with reference methods like MIC) to confirm their identity (e.g., S. aureus) and their methicillin resistance status.

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