(73 days)
The ProGastro™ Cd Assay is a Real Time PCR in vitro diagnostic test for the qualitative detection of toxigenic Clostridium difficile nucleic acids isolated and purified from liquid or soft stool specimens obtained from symptomatic patients. This test targets the Clostridium difficile toxin B gene (tcdB) and is intended for use to aid in the diagnosis of toxigenic Clostridium difficile infections.
The ProGastro Cd Assay detects toxigenic Clostridium difficile and an Internal Control by a process of nucleic acid extraction from patient specimens followed by PCR amplification and detection. Following collection of a soft or liquid stool sample from a symptomatic patient, a portion of the sample is diluted in Stool Transport and Recovery (S.T.A.R.) Buffer and the solids separated via centrifugation (Stool Clarification). The Internal Control is added to the sample prior to extraction to monitor for PCR inhibitors that may be present. The nucleic acids from the sample are extracted and purified using the bioMérieux NucliSENS easyMAG automated extractor. Nucleic acids are added to the C. diff Mix for subsequent PCR amplification and detection using the Cepheid SmartCycler II.
The C. diff Mix contains oligonucleotide primers and probes that target the tcdB gene of toxigenic strains of C. diff. The probes are dual-labeled with a reporter dye attached to the 5'-end and a quencher dye attached to the 3'-end (see table below). During PCR amplification the primers and probes anneal to the template (if present) followed by primer extension and template amplification. The 5'-3' exonuclease activity of the Taq polymerase cleaves the probe thus separating the reporter dye from the quencher and generating an increase in fluorescent signal. The amount of fluorescence at any given cycle is dependent on the amount of amplification product present. The SmartCycler II instrument and software monitors the process, interprets the data, and presents a report upon completion.
Here’s a breakdown of the acceptance criteria and the study proving the ProGastro Cd Assay meets them, based on the provided document:
1. Acceptance Criteria and Reported Device Performance
The document does not explicitly state pre-defined acceptance criteria for Sensitivity and Specificity in a table format. However, the reported performance against the reference method (Tissue Culture Cytotoxin Assay - CTA) implies an expectation of high diagnostic accuracy. For reproducibility, a high percentage of agreement with expected results across sites and operators is an implicit criterion.
Based on the provided data, we can infer the following:
| Performance Metric | Acceptance Criteria (Implied) | Reported Device Performance |
|---|---|---|
| Clinical Study | ||
| Sensitivity | High sensitivity (e.g., >80-90%) compared to CTA | 91.7% (95% CI: 83.0% - 96.1%) |
| Specificity | High specificity (e.g., >90%) compared to CTA | 94.7% (95% CI: 92.8% - 96.1%) |
| Reproducibility | ||
| Overall Agreement | High overall agreement with expected results (e.g., >95%) | 99.0% (95% CI: 97.1% - 99.7%) for primary reproducibility study |
| Agreement for Intermediate Samples | Expected to be in a specific range (e.g., 5-95% positive) due to low concentration | 42.2% (95% CI: 32.5% - 52.5%) for intermediate concentration |
2. Sample Size and Data Provenance
- Sample Size for Test Set: A total of 771 raw stool samples were tested in the clinical performance study.
- Data Provenance: The study was a prospective study conducted at 3 U.S. clinical laboratories from July through October 2008. The samples were "leftover raw stool specimens that were collected for routine Clostridium difficile testing from patients over two years of age by each site." This indicates real-world clinical samples from a U.S. patient population.
3. Number of Experts and Qualifications for Ground Truth
The document does not specify the number of experts used or their qualifications for establishing the initial ground truth (Tissue Culture Cytotoxin Assay - CTA). CTA is a laboratory-based method, and its interpretation would typically be performed by trained laboratory personnel.
4. Adjudication Method for the Test Set
A discrepant analysis was performed for samples where the ProGastro Cd Assay and CTA results disagreed. This involved a predetermined algorithm including:
- A molecular (PCR) test targeting a different region of the tcdB gene.
- Bidirectional genetic sequencing.
- Enzyme Immunoassay (EIA).
- Culture followed by PCR and bidirectional sequencing.
This constitutes a form of adjudication where additional, more definitive tests are used to resolve disagreements between the device and the primary reference method.
5. Multi-Reader Multi-Case (MRMC) Comparative Effectiveness Study
No MRMC comparative effectiveness study was done. This study evaluates an in vitro diagnostic (IVD) test, where the "reader" is essentially the instrument and the assay, not a human interpreter of an image or signal. Therefore, assessing how human readers improve with AI vs. without AI assistance is not applicable here.
6. Standalone (Algorithm-Only) Performance
The entire clinical performance study report (Sensitivity, Specificity) evaluates the standalone performance of the ProGastro Cd Assay. The assay is an in vitro diagnostic test for the qualitative detection of C. difficile nucleic acids, which runs entirely on the Cepheid SmartCycler II instrument and software for amplification, detection, and data interpretation. There is no human-in-the-loop component for the result generation itself.
7. Type of Ground Truth Used
The primary reference method used to establish ground truth for the clinical performance study was the Tissue Culture Cytotoxin Assay (CTA). For discrepant analysis, additional molecular tests (PCR, sequencing), EIA, and culture were used to refine the ground truth. This combines a gold standard (CTA) with a more comprehensive "adjudicated" gold standard for discordant results.
8. Sample Size for the Training Set
The document does not provide information about a specific training set or its sample size. This is typical for 510(k) submissions for diagnostic assays, where the focus is on the performance of the finalized device rather than the development process involving training data for algorithms. The "assay" itself, like most IVDs, is developed and validated, but does not typically undergo a 'training' phase with a specific dataset in the same way a machine learning algorithm would.
9. How Ground Truth for the Training Set Was Established
As no specific training set information is provided, the method for establishing its ground truth is also not described.
§ 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.