(84 days)
The Cepheid Xpert® C. difficile Assay, performed on the Cepheid GeneXpert® Dx System, is a qualitative in vitro diagnostic test for rapid detection of toxin B gene sequences from unformed (liquid or soft) stool specimens collected from patients suspected of having Clostridium difficile infection (CDI). The test utilizes automated real-time polymerase chain reaction (PCR) to detect toxin gene sequences associated with toxin producing C. difficile. The Xpert C. difficile Assay is intended as an aid in the diagnosis of CDI. Concomitant culture is necessary only if further typing or organism recovery is required.
The Cepheid Xpert C. difficile Assay is a rapid, automated in vitro diagnostic test for qualitative detection of Clostridium difficile directly from unformed (liquid or soft) stool specimens of patients suspected of having Clostridium difficile infection (CDI). The assay detects the toxin B gene (tcdB), and is performed on the Cepheid GeneXpert Dx System.
The Xpert C. difficile Assay system performs sample preparation and real-time, multiplex polymerase chain reaction (PCR) for detection of target-specific DNA.
The GeneXpert Dx System consists of a GeneXpert® instrument, personal computer, and disposable fluidic cartridges. Each instrument contains 1-16 randomly accessible modules that are each capable of performing separate sample preparation and real-time PCR tests for detection of C. difficile toxin B gene sequences in less than 45 minutes. Each module contains a syringe drive for dispensing fluids, an ultrasonic horn for lysing cells or spores, and I-CORE® thermocycler for performing real-time PCR and detection.
A swab is inserted into the stool specimen and then is placed in a tube containing elution reagent. Following brief vortexing, the eluted material and two single-use reagents (Reagent 1 and Reagent 2) that are provided with the Assay are transferred to different, uniquely-labeled chambers of the disposable fluidic cartridge (the Xpert C. difficile cartridge). The user initiates a test from the system user interface and places the cartridge into the GeneXpert® Dx System instrument platform, which performs hands-off realtime, multiplex polymerase chain reaction (PCR) for detection of DNA. In this platform, additional sample preparation, amplification, and real-time detection are all fullyautomated and completely integrated.
The Xpert C. difficile Assay includes reagents for the detection of toxin B gene (tcdB). In addition, the assay reagents include an internal sample processing control (SPC) to ensure adequate processing of the target bacteria and to monitor the presence of inhibitor(s) in the PCR Assay. The SPC also ensures that the PCR conditions (temperature and time) are appropriate for the amplification reaction and that the PCR reagents are functional. The Probe Check Control (PCC) verifies reagent rehydration, PCR tube filling in the cartridge, probe integrity, and dye stability.
Here's a summary of the acceptance criteria and the study details for the Cepheid Xpert C. difficile Assay, based on the provided document:
1. Table of Acceptance Criteria and Reported Device Performance:
The document doesn't explicitly state "acceptance criteria" with numerical thresholds in a dedicated table format. Instead, it describes the performance observed in various studies. Based on the "Overall Results" and "Performance vs. Reference Culture" sections of the clinical study, the following can be inferred as the key performance metrics evaluated and achieved:
| Acceptance Criteria (Implied) | Reported Device Performance (Xpert C. difficile Assay vs. Reference Culture) |
|---|---|
| High Sensitivity for Toxigenic C. difficile | 93.49% |
| High Specificity for Toxigenic C. difficile | 94.02% |
| High Accuracy for Toxigenic C. difficile | 93.95% |
| Acceptable Positive Predictive Value (PPV) for Toxigenic C. difficile | 72.98% |
| High Negative Predictive Value (NPV) for Toxigenic C. difficile | 98.82% |
| Reproducible Results (Total Agreement) | 98.1% (across 3 sites, all samples; for specific low positive samples, it was 90.0% and 96.7%) |
| Analytical Inclusivity (detection of diverse C. difficile strains) | 100% (all 13 tested toxinotypes correctly identified) |
| Analytical Sensitivity / Limit of Detection (LoD) | Positively detects C. difficile 95% of the time with 95% confidence for a fecal sample containing 460 CFU. Lower LoD observed for specific toxinotypes (e.g., 23 CFU/swab for LUMC-1). |
| Analytical Specificity (no cross-reactivity) | 100% (all 55 non-C. difficile strains/species correctly reported as negative) |
| No significant interference from common substances | 19 tested substances showed no detectable interference |
2. Sample Size Used for the Test Set and Data Provenance:
- Sample Size for the Clinical Study (Test Set): A total of 2296 specimens were tested.
- Data Provenance: The study was a "multisite prospective investigation study at seven US and Canadian institutions." This indicates a prospective design with data collected from multiple locations across the US and Canada.
3. Number of Experts Used to Establish the Ground Truth for the Test Set and Their Qualifications:
The document describes the ground truth for the clinical study as "reference culture followed by cell cytotoxicity testing on the isolates." It does not explicitly mention the number of experts or their specific qualifications (e.g., "radiologist with 10 years of experience") involved in performing and interpreting these reference methods. The reference culture and cytotoxin B isolate testing would typically be performed and interpreted by trained laboratory personnel or microbiologists.
4. Adjudication Method for the Test Set:
The document does not describe a formal "adjudication method" involving multiple expert readers for the ground truth. The ground truth was established by the "reference culture method followed by cell cytotoxicity testing on the isolates." This implies a definitive laboratory result rather than a subjective interpretation requiring adjudication among human readers.
5. If a Multi-Reader Multi-Case (MRMC) Comparative Effectiveness Study was done, and its effect size:
No, a Multi-Reader Multi-Case (MRMC) comparative effectiveness study was not done. This study compared the device (Xpert C. difficile Assay) directly against a laboratory reference standard (culture and cytotoxicity testing), not against human readers with and without AI assistance.
6. If a Standalone (i.e. algorithm only without human-in-the-loop performance) was done:
Yes, the studies described are standalone performance evaluations of the Xpert C. difficile Assay. The device is a "qualitative in vitro diagnostic test for rapid detection of toxin B gene sequences... The test utilizes automated real-time polymerase chain reaction (PCR) to detect toxin gene sequences." The results presented (sensitivity, specificity, LoD, etc.) reflect the performance of the automated algorithm and system without human interpretation as part of the primary diagnostic output. Human involvement is limited to specimen collection, preparation (swab elution, reagent transfer), and initiating the test on the GeneXpert Dx System.
7. The Type of Ground Truth Used:
The primary ground truth used for the clinical comparison study was:
- Reference culture method followed by cell cytotoxicity testing on the isolates.
Specifically:
- Stool specimens were inoculated onto CCFA-D and CCMB-TAL.
- If C. difficile was isolated from CCFA-D and positive by cell cytotoxicity assay, it was classified as "toxigenic C. difficile positive."
- If not, further analysis was done on CCFA-E (subcultured from CCMB-TAL).
- If CCFA-E was positive for C. difficile and the isolate positive for cell cytotoxicity assay, it was classified as "toxigenic C. difficile positive."
- Otherwise, it was reported as "negative."
8. The Sample Size for the Training Set:
The document does not explicitly mention a separate "training set" or its sample size. The focus is on the analytical and clinical validation of the device. Diagnostic PCR assays typically do not have a "training set" in the same way machine learning algorithms do. Instead, they are developed and optimized (which could be considered analogous to training) using various strains and concentrations during the research and development phase. The analytical inclusivity, sensitivity (LoD), and specificity studies describe the validation of the device's ability to detect different strains and concentrations.
9. How the Ground Truth for the Training Set was Established:
As there is no explicitly defined "training set" in the context of this document, there's no description of how its ground truth was established. For the analytical studies, the ground truth was established by:
- Known characteristics of bacterial strains: Analytical inclusivity used 13 C. difficile strains of different toxinotypes, with their toxinotype status being the known ground truth.
- Known concentrations: Analytical sensitivity (LoD) involved preparing C. difficile at known CFU concentrations in a fecal matrix, with these known concentrations serving as the ground truth.
- Confirmed identity of microorganisms: Analytical specificity used 55 strains (including non-toxigenic C. difficile and other Clostridium species) whose identities were confirmed from reputable culture collections or institutions, acting as the ground truth for their presence or absence.
§ 866.2660 Microorganism differentiation and identification device.
(a)
Identification. A microorganism differentiation and identification device is a device intended for medical purposes that consists of one or more components, such as differential culture media, biochemical reagents, and paper discs or paper strips impregnated with test reagents, that are usually contained in individual compartments and used to differentiate and identify selected microorganisms. The device aids in the diagnosis of disease.(b)
Classification. Class I (general controls). The device is exempt from the premarket notification procedures in subpart E of part 807 of this chapter subject to § 866.9.