(72 days)
The Quidel Molecular Direct C. difficile Assay is a qualitative, multiplexed in vitro diagnostic test for the direct detection of toxin A gene (tcdA) or toxin B gene (tcdB) sequences of toxigenic strains of Clostridium difficile from unformed (liquid or soft) stool specimens collected from patients suspected of having Clostridium difficile-Associated Disease (CDAD).
The Quidel Molecular Direct C. difficile Assay is a real-time PCR test and utilizes proprietary sample preparation with fluorescently labeled primers and probes. The assay can be performed using either the Life Technologies QuantStudio® Dx; the Applied Biosystems 7500 Fast Dx, or the Cepheid SmartCycler II, to detect the toxin gene sequences associated with toxin-producing C. difficile strains.
The assay is intended to be performed directly on CDAD-suspected stool specimens, and is indicated for use as an aid in the diagnosis of CDAD.
The Quidel Molecular Direct C. difficile Assay detects nucleic acids that have been prepared from a patient sample using proprietary sample preparation. A multiplex real-time PCR reaction is performed under optimized conditions in a single well generating amplicons for each of the targets present in the sample. Identification occurs by the use of oligonucleotide primers and probes that are complementary to conserved regions in the tcdA and tcdB genes of the pathogenicity locus (PaLoc).
The Quidel Molecular Direct C. difficile Assay contains sufficient reagents to process 96 specimens or quality control samples.
Here's a summary of the acceptance criteria and study details for the Quidel Molecular Direct C. difficile Assay, based on the provided document:
1. Table of Acceptance Criteria and Reported Device Performance
The document primarily focuses on demonstrating the device's performance against de facto clinical comparison methods rather than explicitly stating pre-defined acceptance criteria with specific numerical thresholds for sensitivity and specificity before the study. However, the reported performance metrics can be considered the demonstrated "acceptance" level based on the study outcomes.
| Metric (Comparison to Reference) | Acceptance/Reported Performance - ABI 7500 Fast Dx | Acceptance/Reported Performance - Life Technologies QuantStudio Dx | Acceptance/Reported Performance - Cepheid SmartCycler II |
|---|---|---|---|
| Clinical Performance (vs. Direct Culture Cytotoxicity Assay) | |||
| Sensitivity (Quidel POS / Ref POS) | 94.3% (95% CI: 87.4% - 97.5%) | 93.3% (95% CI: 86.9% - 96.7%) | 89.7% (95% CI: 81.5% - 94.5%) |
| Specificity (Quidel NEG / Ref NEG) | 94.2% (95% CI: 91.9% - 95.8%) | 93.4% (95% CI: 91.3% - 95.0%) | 93.3% (95% CI: 91.0% - 95.1%) |
| Clinical Performance (vs. Enriched Toxigenic Culture) | |||
| Sensitivity (Quidel POS / Ref POS) | 88.9% (95% CI: 82.2% - 93.3%) | 87.3% (95% CI: 81.1% - 91.6%) | 82.4% (95% CI: 74.8% - 88.1%) |
| Specificity (Quidel NEG / Ref NEG) | 98.9% (95% CI: 97.6% - 99.5%) | 98.7% (95% CI: 97.5% - 99.4%) | 97.9% (95% CI: 95.4% - 99.1%) |
| Analytical Sensitivity (Limit of Detection - LoD) | |||
| ATCC BAA-1870 (IIIb) | 8.4E+04 CFU/mL (4.2E-01 CFU/Assay) | 8.4E+04 CFU/mL (4.2E-01 CFU/Assay) | 8.4E+04 CFU/mL (4.2E-01 CFU/Assay) |
| ATCC BAA-1872 (0) | 2.4E+04 CFU/mL (1.2E-01 CFU/Assay) | 8.0E+03 CFU/mL (4.0E-02 CFU/Assay) | 2.4E+04 CFU/mL (1.2E-01 CFU/Assay) |
| Analytical Reactivity (Inclusivity) | Detected at 2-3x LoD for various toxigenic strains | Detected at 2-3x LoD for various toxigenic strains | Detected at 2-3x LoD for various toxigenic strains |
| Analytical Specificity (Cross-Reactivity) | No cross-reactivity with tested microorganisms and human DNA | No cross-reactivity with tested microorganisms and human DNA | No cross-reactivity with tested microorganisms and human DNA |
| Interfering Substances | No interference observed with 35 substances | No interference observed with 35 substances | No interference observed with 35 substances |
| Precision (% Detection at 2X LoD) | 100% | 100% | 96% |
| Reproducibility (Overall % Detection at 2X LoD) | 88/90 (97.8%) | 90/90 (100%) | 88/89 (98.9%) |
2. Sample Size Used for the Test Set and Data Provenance
- Clinical Performance Studies (for ABI 7500 Fast Dx & Cepheid SmartCycler II): 665 specimens initially, reduced to 653 (vs Direct Culture) or 656 (vs Enriched Culture) due to invalid/indeterminate results.
- Clinical Performance Study (for Life Technologies QuantStudio Dx): 792 specimens initially, reduced to 788 (vs Direct Culture) or 791 (vs Enriched Culture) due to invalid/indeterminate results.
- Provenance: All clinical specimens were collected prospectively between August 2012 and November 2012 from patients suspected of having Clostridium difficile-Associated Disease (CDAD) at four geographically diverse locations within the United States.
3. Number of Experts Used to Establish the Ground Truth for the Test Set and Their Qualifications
The document does not explicitly mention the use of human experts to establish the ground truth for the clinical test set. Instead, the ground truth was established by laboratory-based reference assays:
- Direct Culture Cytotoxicity Assay: This is a biological assay, not directly performed by "experts" in the sense of clinical reviewers.
- Enriched Toxigenic C. difficile Culture: This is also a laboratory culture method, not involving expert review of images or clinical data.
- FDA-cleared molecular device: Used for discordant analysis, this is another laboratory test.
Therefore, the concept of "experts" and their qualifications as typically applied to image-based diagnostic devices (e.g., radiologists) does not apply here.
4. Adjudication Method for the Test Set
The document describes an adjudication method for discordant results.
- For discordant results between the Quidel Molecular Direct C. difficile Assay and the reference methods (Direct Culture Cytotoxicity Assay or Enriched Toxigenic Culture), an FDA-cleared molecular device was used to re-test the discordant specimens. The results from this FDA-cleared molecular device were then used to help explain the discrepancies.
- The overall clinical performance was calculated based on the initial test results prior to the discordant analysis.
5. Multi-Reader Multi-Case (MRMC) Comparative Effectiveness Study
No, a Multi-Reader Multi-Case (MRMC) comparative effectiveness study was not conducted. This type of study typically involves multiple human readers assessing cases with and without AI assistance to measure the effect size of AI on human performance. The Quidel Molecular Direct C. difficile Assay is a qualitative PCR test, not an imaging-based diagnostic system that would involve human interpretation of images, so an MRMC study is not relevant.
6. Standalone (Algorithm Only Without Human-in-the-Loop Performance) Study
Yes, the primary clinical performance studies (comparison against Direct Culture Cytotoxicity Assay and Enriched Toxigenic Culture) represent a standalone (algorithm only) performance evaluation of the Quidel Molecular Direct C. difficile Assay. The assay directly reports a qualitative result (positive or negative) without requiring human interpretation or involvement in the diagnostic decision once the sample is processed by the instrument.
7. The Type of Ground Truth Used
The ground truth for the clinical performance studies was established using laboratory reference methods:
- Direct Culture Cytotoxicity Assay: This method identifies the presence of C. difficile toxins.
- Enriched Toxigenic C. difficile Culture: This method specifically identifies toxigenic strains of C. difficile.
For discordant analyses, an FDA-cleared molecular device was used as a third reference.
8. The Sample Size for the Training Set
The document does not specify a training set sample size. This is common for molecular diagnostic assays like this one, where the assay design (e.g., primer and probe sequences) is developed based on known genetic sequences of the target organism, and analytical validation (LoD, inclusivity, specificity) is performed. The clinical studies presented are for validation/testing, not for training a machine learning model.
9. How the Ground Truth for the Training Set Was Established
As there is no explicitly mentioned "training set" in the context of machine learning for this device, the concept of ground truth for a training set is not directly applicable. The assay's design and analytical characteristics are based on:
- Knowledge of conserved regions in C. difficile tcdA and tcdB genes.
- Quantified cultures of C. difficile strains (ATCC BAA-1870 and ATCC BAA-1872) for analytical sensitivity (LoD).
- Known toxigenic strains for analytical reactivity (inclusivity).
- A panel of known bacterial, viral, and yeast microorganisms, as well as human DNA, for analytical specificity (cross-reactivity).
§ 866.3130 Clostridium difficile toxin gene amplification assay.
(a)
Identification. AClostridium difficile toxin gene amplification assay is a device that consists of reagents for the amplification and detection of target sequences inClostridium difficile toxin genes in fecal specimens from patients suspected of havingClostridium difficile infection (CDI). The detection of clostridial toxin genes, in conjunction with other laboratory tests, aids in the clinical laboratory diagnosis of CDI caused byClostridium difficile. (b)
Classification. Class II (special controls). The special controls are set forth in FDA's guideline document entitled: “Class II Special Controls Guideline: Toxin Gene Amplification Assays for the Detection ofClostridium difficile; Guideline for Industry and Food and Drug Administration Staff.” See § 866.1(e) for information on obtaining this document.