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The KeyPath™ MRSA/MSSA Blood Culture Test - BT is a qualitative in vitro diagnostic test for the timely identification of Staphylococcus aureus (S. aureus) and determination of methicillin susceptibility (MSSA) or methicillin resistance (MRSA) directly from positive blood cultures.

The Test uses bacteriophage amplification to identify the presence of S. aureus and assess the phenotypic response of the target organism to cefoxitin, an indicator of oxacillin (a methicillin analog) resistance.

The assay is performed directly on positive blood culture specimens that are determined as Gram Positive Cocci in singles (GPC) or as Gram Positive Cocci in Clusters (GPCC) by Gram stain.

The KeyPath™ MRSA/MSSA Blood Culture Test - BT is performed directly on positive blood culture specimens from BD BACTEC™ blood culture bottles (Plus Aerobic/F and Plus Anaerobic/F).

The Test is indicated for use in conjunction with other laboratory and clinical data available to the physician as an aid in the detection of MRSA/MSSA from positive blood cultures.

Subculturing of positive blood cultures is necessary for additional susceptibility test determinations, differentiation of mixed growth and for epidemiological typing.

The KeyPath™ MRSA/MSSA Blood Culture Test – BT uses lytic bacteriophage, specific for Staphylococcus aureus, as an amplification technology for detection of S. aureus and determination of methicillin resistance or susceptibility in positive blood cultures. To detect S. aureus (ID Reaction Tube), the bacteriophage infect the S. aureus (if present), replicate within the host (culminating in bacterial lysis) and over the incubation period, produce several cvcles of bacteriophage amplification. In a separate Reaction Tube (RS), the Test uses cefoxitin (an oxacillin and methicillin analog) which inhibits bacteriophage amplification for susceptible organisms (MSSA) and fails to inhibit bacteriophage amplification when the organism is resistant to methicillin (MRSA). The Test then uses a self-performing immunoassay (Detector) to detect the increase in concentration of bacteriophage using antibodies specific to the Test bacteriophage, and calibrated such that at above a threshold concentration, it produces a visible signal.

Here's a breakdown of the acceptance criteria and study details for the MicroPhage KeyPath™ MRSA/MSSA Blood Culture Test - BT, based on the provided document:

1. Table of Acceptance Criteria and Reported Device Performance

The acceptance criteria are not explicitly stated as numerical targets in the document. Instead, the study aims to achieve substantial equivalence to predicate devices and provide robust performance characteristics. The reported device performance is listed below for different analytical measures.

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

• Test Set Sample Size: 1116 paired samples (366 S. aureus).
• Data Provenance: The study was a prospective clinical trial conducted across four clinical sites. The country of origin of the data is not explicitly stated, but given the FDA 510(k) submission, it is typically from the United States or an equivalent regulatory region.

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

The document does not specify the number of experts directly, nor their specific qualifications. However, the "standard methods" used to establish ground truth imply the involvement of trained laboratory personnel. These methods include:

• Standard culture identification for S. aureus (Catalase positive, Tube Coagulase positive, Remel Staphaurex® positive).
• Antimicrobial susceptibility determination using 30 µg cefoxitin disk diffusion in accordance with CLSI M100-S19.

This suggests that the ground truth was established by experienced clinical laboratory professionals following established microbiological and susceptibility testing guidelines.

4. Adjudication Method for the Test Set

The document does not explicitly describe an adjudication method for discrepancies between the KeyPath™ Test results and the "standard methods." It notes that invalid results from the KeyPath™ Test were "resolved upon retest," implying re-evaluation of those specific samples for the KeyPath™ Test, but not a formal adjudication of ground truth discrepancies.

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, an MRMC comparative effectiveness study involving human readers and AI assistance was not mentioned or performed for this device. The KeyPath™ MRSA/MSSA Blood Culture Test - BT is a diagnostic test kit involving a visual interpretation from a lateral flow immunoassay, not an AI-powered image analysis or diagnostic assist tool for human readers.

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

Yes, the study primarily assesses the standalone performance of the KeyPath™ MRSA/MSSA Blood Culture Test - BT device as a diagnostic test. While the interpretation of the lateral flow immunoassay is visual, it's the test kit's performance being evaluated against established gold standards, not a human reader's performance with or without the kit.

7. The Type of Ground Truth Used (Expert Consensus, Pathology, Outcomes Data, etc.)

The ground truth used was a combination of established microbiological culture methods and phenotypic antimicrobial susceptibility testing:

• For S. aureus identification: Catalase test, Tube Coagulase test, and Remel Staphaurex® test.
• For methicillin resistance/susceptibility: 30 µg cefoxitin disk diffusion test, performed according to CLSI M100-S19 guidelines.

This represents a robust and well-accepted gold standard in clinical microbiology.

8. The Sample Size for the Training Set

The document does not explicitly mention a separate "training set" or its size for the KeyPath™ MRSA/MSSA Blood Culture Test - BT. This device is a diagnostic test kit that uses bacteriophage amplification and immunoassay detection, which typically involves wet-lab development and validation rather than machine learning models that require distinct training sets. The "Non-clinical Studies" section describes inclusivity and analytical specificity testing on panels of strains, which could be considered part of the development and refinement process, but not a "training set" in the context of algorithm development.

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

As no specific "training set" for an algorithm is mentioned, this question is not directly applicable. However, for the panels used in non-clinical studies (e.g., inclusivity, analytical specificity), the ground truth for strain identification and resistance profiles would have been established using comprehensive microbiological techniques and genetic characterization in a laboratory setting. For instance, the "Evaluation of SCCmec Empty Cassette Variants" section mentions "mecA-positive (MRSA) by commercial and laboratory PCR tests but MSSA by phenotypic culture." This indicates that molecular methods (PCR) and traditional culture methods were used to characterize these strains.

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