(213 days)
mecA XpressFISH® is a qualitative nucleic acid fluorescence in situ hybridization assay intended for the detection of mecA mRNA on smears from blood cultures that are positive for Staphylococcus aureus by the Staphylococcus QuickFISH™ BC assay.
The mecA XpressFISH® assay is indicated for use in conjunction with other laboratory tests and clinical data available to the clinician as an aid in the detection of mecA mRNA from methicillin-resistant S. aureus (MRSA) from patient positive blood cultures. The mecA XpressFISH® assay is not intended to monitor treatment for MRSA infections or for use with mixed cultures including those containing both S. aureus and coagulase negative staphylococci.
Sub-culturing of positive blood cultures is necessary to recover organisms for susceptibility testing, epidemiological typing, and/or differentiation of mixed growth.
The mecA XpressFISH assay is a qualitative fluorescence in situ hybridization (FISH) assay which utilizes peptide nucleic acid (PNA) probes hybridizing to mecA messenger RNA (mRNA) on smears from blood cultures containing Staphylococcus aureus (SA). After the positive blood culture bottle is confirmed to have SA by Staphylococcus QuickFISH BC, an aliquot of the blood culture is incubated with media containing cefoxitin to induce mecA mRNA expression prior to mecA XpressFISH testing.
The assay consists of mecA mRNA inducing cefoxitin reagents, a fixation reagent, a hybridization reagent, a wash solution concentrate, coverslip mounting media and a microscope slide which has a sample well, and positive and negative control wells. Results are visualized and interpreted using fluorescence microscopy.
Here's an analysis of the provided text, extracting the acceptance criteria and study details for the mecA XpressFISH device:
The document describes the mecA XpressFISH assay, a qualitative nucleic acid fluorescence in situ hybridization assay for the detection of mecA mRNA in Staphylococcus aureus (SA) positive blood cultures. The goal is to identify methicillin-resistant S. aureus (MRSA).
1. Table of Acceptance Criteria and Reported Device Performance
The acceptance criteria are generally implied through the comparability with the predicate device (BinaxNOW PBP2a) and the detailed validation studies demonstrating high agreement, sensitivity, and specificity. Explicit, numerical acceptance criteria for each bench test are not always stated, but the results consistently show 100% agreement or very high percentages, indicating successful performance against internal or industry-standard benchmarks.
| Acceptance Criteria (Implied/Derived) | Reported Device Performance |
|---|---|
| Clinical Performance: | |
| High Agreement with Predicate Device (Cefoxitin Disk Diffusion) for MRSA Identification | 99.1% (336/339) overall agreement with cefoxitin disk diffusion in clinical studies. |
| High Sensitivity for MRSA Detection | 98.7% (151/153), 95% CI (95.4-99.6) |
| High Specificity for MRSA Detection | 99.5% (185/186), 95% CI (97.0-99.9) |
| Positive Predictive Value (PPV) | 99.3% (151/152), 95% CI (96.4-99.9) |
| Negative Predictive Value (NPV) | 98.9% (185/187), 95% CI (96.2-99.7) |
| Bench Testing (Functional Requirements): | |
| Limit of Detection (LoD) comparable to other slide-based tests | Lowest readable concentration for MRSA strains: 8.1x10^5 - 2.8x10^6 CFU/mL (comparable to Staphylococcus QuickFISH and Gram Stain). |
| Analytic Reactivity (Inclusivity) for MRSA strains | 63/64 MRSA strains produced Green Positive results (1 strain known not to carry mecA did not). |
| Analytic Specificity (Exclusivity) against non-MRSA organisms | 29/30 MSSA produced Negative results (1 mecA+ MSSA gave Green Positive). 34/40 other CoNS produced Negative results (6 produced Green Positive). 36/37 other Gram-positive were Negative (Lactococcus lactis Green Positive). 18/19 other clinically relevant species were Negative (Candida parapsilosis Green Positive). |
| No significant interference from co-infecting species | No False Positive results from methicillin-sensitive staphylococci; no interference observed in MRSA detection. |
| Robustness against various interfering substances | All MRSA tests produced Positive results (except weak positive with Linezolid, noted as limitation). All MSSA strains showed Negative results. Overall acceptable performance. |
| Compatibility with common blood culture media | 100% agreement for all 9 tested media types (MRSA Positive, MSSA Negative). |
| Absence of cross-contamination during wash step | 100% of MRSA samples Green Positive, 100% of MSSA samples Negative; no Green Positive in MSSA wells. |
| Effective Cefoxitin concentration range for mecA induction | 100% of 20 MRSA strains Positive (60/60); 100% of 3 MSSA strains Negative (9/9) across concentrations 2.5, 6.1, and 15 µg/mL. |
| Effective Cefoxitin induction time range | 100% of 20 MRSA strains Positive (60/60); 100% of 3 MSSA strains Negative (9/9) across 40, 45, 50 minutes. |
| Tolerance for Induction to Slide Preparation Interval | 100% of 20 MRSA strains Positive (60/60); 100% of 3 MSSA strains Negative (9/9) across immediate, 30 min, 60 min delays. |
| Stability of Fixed Slides | 100% of 158 MRSA strains Positive; 100% of 23 MSSA strains Negative across various storage conditions and times. |
| Effective Hybridization Time Range | 100% of 20 MRSA strains Positive (60/60); 100% of 3 MSSA strains Negative (9/9) across 10, 15, 20 minutes. |
| Effective Slide Wash Time Range | 100% of 20 MRSA strains Positive (60/60); 100% of 3 MSSA strains Negative (9/9) across 10, 15, 20 minutes. |
| Performance maintained across age of blood culture | 100% of 20 MRSA strains Positive (80/80); 100% of 3 MSSA strains Negative (12/12) across bottle ring to ~60 hours. (One MRSA strain showed weak signal beyond 6-12 hr). |
| Reproducibility of results (intra-run, inter-run, inter-site, inter-operator) | 100% Positive Green Agreement (120/120) and 100% Negative Agreement (60/60) across 3 sites, 2 operators, 5 days, and two different concentrations for 2 MRSA and 1 MSSA strain. |
2. Sample Sizes Used for the Test Set and Data Provenance
- Clinical Study Test Set Sample Size: 339 routine blood culture bottles that were positive for Staphylococcus aureus by Staphylococcus QuickFISH BC.
- Data Provenance: The clinical study was conducted at seven clinical sites (6 in the US and 1 ex-US). This indicates a multi-center, prospective clinical study collecting real-world samples.
3. Number of Experts Used to Establish the Ground Truth for the Test Set and Qualifications of Those Experts
The ground truth for the clinical study was established using cefoxitin disk diffusion, which is a widely accepted laboratory method for determining methicillin resistance in clinical microbiology. The document does not specify the number or qualifications of experts who performed or interpreted the cefoxitin disk diffusion tests. It is assumed these were performed by trained laboratory personnel following standard clinical microbiology guidelines.
4. Adjudication Method for the Test Set
The document does not explicitly describe an adjudication method for the test set in the clinical study beyond stating the comparison was made against cefoxitin disk diffusion. Discrepant results are noted and briefly analyzed (e.g., "False Positive mecA (weak Green Positive); Negative upon repeat testing. Cefoxitin disk=28 mm."), suggesting individual review of such cases, but a formal adjudication process (e.g., 2+1, 3+1 consensus) is not detailed.
5. Multi-Reader Multi-Case (MRMC) Comparative Effectiveness Study
- No, a multi-reader multi-case (MRMC) comparative effectiveness study comparing human readers with AI assistance versus human readers without AI assistance was not performed or described in this document.
- The device is a diagnostic assay interpreted visually by a single operator, not an AI-assisted diagnostic.
6. Standalone (Algorithm Only Without Human-in-the-Loop Performance) Study
- The mecA XpressFISH assay is a standalone diagnostic assay in the context that its results are interpreted by an operator viewing fluorescence microscopy images. It is not an "algorithm only" device in the AI sense.
- The "standalone" performance here refers to the direct output of the assay itself, interpreted by a human, without further AI algorithms altering or assisting that interpretation. The clinical study precisely evaluates this standalone performance.
7. Type of Ground Truth Used
- Clinical Study Ground Truth: Cefoxitin Disk Diffusion for identification of methicillin-resistant S. aureus (MRSA). This is a well-established phenotypic method used in clinical microbiology as a standard of care for resistance testing.
- Bench Testing Ground Truth: Laboratory-characterized strains (e.g., ATCC strains, well-characterized clinical isolates) with known mecA gene status or methicillin resistance phenotypes were used for inclusivity, exclusivity, LOD, and reproducibility studies.
8. Sample Size for the Training Set
- The document does not specify a distinct "training set" in the context of machine learning.
- This device is a traditional in vitro diagnostic (IVD) assay, not an AI/ML-based device that would typically have a separate training set. Development and internal bench testing used various characterized strains, but these do not constitute a "training set" in the AI sense.
9. How the Ground Truth for the Training Set Was Established
- As there isn't a "training set" in the AI sense, this question isn't directly applicable.
- For the development and bench marking activities, ground truth was established through:
- Using reference strains (e.g., ATCC strains with known characteristics).
- Characterization of clinical isolates using standard microbiological methods to determine their mecA status or methicillin resistance profile.
§ 866.1640 Antimicrobial susceptibility test powder.
(a)
Identification. An antimicrobial susceptibility test powder is a device that consists of an antimicrobial drug powder packaged in vials in specified amounts and intended for use in clinical laboratories for determining in vitro susceptibility of bacterial pathogens to these therapeutic agents. Test results are used to determine the antimicrobial agent of choice in the treatment of bacterial diseases.(b)
Classification. Class II (performance standards).