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Remel Xpect® Flu A&B is a rapid in vitro immunochromatographic test for the direct, qualitative detection of influenza A and influenza B viral antigens (nucleoprotein) from nasal wash, nasal swab, and throat swab specimens from symptomatic patients. The test is intended as an aid in the rapid diagnosis of influenza A and influenza B viral infections. Negative tests should be confirmed by cell culture.

The Xpect® Flu A&B is a chromatographic immunoassay for the qualitative detection of influenza A and influenza B viral antigens. The test device incorporates separate membrane strips for influenza A and for influenza B. To perform the test, the patient specimen is diluted and added to the sample wells of the device. The mixture moves along the membranes by capillary action. If present, influenza A or B viral antigens in the patient sample bind anti-influenza A or B conjugated antibodies. A visible line forms as a complex of antibodyantigen-antibody coated colored particles is captured in the test region (T). Antibody coated colored particles not bound at the test line are later captured in the control region (C) containing goat anti-mouse antibody. A visible line will always appear in the control region indicating that the test is working properly. The presence of a control line combined with the absence of a visible test line is interpreted as a negative test result.

Here's an analysis of the provided text regarding the acceptance criteria and the study that proves the device meets them:

1. Table of Acceptance Criteria & Reported Device Performance

The provided text describes a 510(k) submission for a device modification (K092423) that expands the reactivity table of an existing device (Remel Xpect® Flu A&B, K031565). The core acceptance criteria revolve around analytical sensitivity, specifically the detection limits for various influenza strains.

The key new data point relevant to this specific 510(k) (K092423) is the inclusion of the A/California/04/2009 (H1N1) strain with a detected limit of 4.41 x 10^2 TCID50/ml.

Other key strains and their detection limits are provided in the "Summary of Performance Data" table within the text, for example:

• A/New Caledonia/20/1999 (H1N1): 1.63 x 10^2 TCID50/ml
• A/Fort Monmouth/1/47 (H1N1): 7.9 x 10^1 CEID50/ml
• A/Hong Kong/8/68 (H3N2): 2.8 x 10^1 CEID50/ml
• B/Allen/45 (B): 4 x 10^0 CEID50/ml |
  | Intended Use: Rapid in vitro immunochromatographic test for direct, qualitative detection of influenza A and B viral antigens from nasal wash, nasal swab, and throat swab specimens from symptomatic patients, as an aid in rapid diagnosis. Negative tests should be confirmed by cell culture. | Intended Use: The intended use of the modified device has not changed from the predicate device and aligns with the acceptance criteria. |
  | Fundamental Scientific Technology: No changes to the fundamental scientific technology. | Fundamental Scientific Technology: The submission explicitly states, "This modification has not had any effect or caused any changes to the FUNDAMENTAL SCIENTIFIC TECHNOLOGY of this device." This was accepted by the reviewer. |
  | Risk Assessment: Address risks of false positive and false negative results, with mitigation through labeling, training, QC, and confirmation of negative results. | Risk Assessment: The study indicates that "The risks of false positive and false negative test results as related to the risks to patients, were addressed with labeling, appropriate training for users, customer quality control and confirmation of all negative results by cell culture." |

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

• Sample Size (Test Set): For the analytical sensitivity study (which serves as the primary test set for this modification), the study used 16 influenza strains (10 influenza A and 6 influenza B). Each strain was quantitated and subjected to serial titration to determine the detection limit. This is a laboratory-based analytical study, not a clinical study with patient samples.
• Data Provenance: The data is from a retrospective analytical study conducted in a laboratory setting. There is no information provided about the country of origin of the data beyond the applicant "Thermo Fisher Scientific Remel Products" being based in Lenexa, KS, USA, implying the study was conducted internally or contracted within the US. The strains themselves are named with geographic locations (e.g., A/California/04/2009, A/Puerto Rico/8/34), but this refers to the strain's isolation origin, not the origin of the study data.

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

• Number of Experts: Not applicable. For this analytical sensitivity study, the "ground truth" for each virus strain's concentration (TCID50/ml or CEID50/ml) would have been established through standardized laboratory methods (e.g., cell culture titration) rather than expert consensus on individual test results.
• Qualifications of Experts: Not applicable. The "ground truth" for viral concentration is determined by quantitative assay rather than human interpretation.

4. Adjudication Method for the Test Set

• Adjudication Method: Not applicable. This was an analytical study determining the detection limit of viral strains. The endpoint was reached when a positive result was consistently observed, implying a clear, objective assessment rather than a need for adjudicated interpretations of ambiguous results.

5. If a Multi-Reader Multi-Case (MRMC) Comparative Effectiveness Study Was Done, and Effect Size

• MRMC Study: No, an MRMC comparative effectiveness study was not conducted. This submission specifically addresses an analytical modification (expanded reactivity) and not a change in the device's human-in-the-loop performance or clinical utility compared to human readers.
• Effect Size: Not applicable as no MRMC study was performed.

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

• Standalone Performance: Yes, the analytical sensitivity study is a standalone performance assessment. The device (Remel Xpect® Flu A&B) is an immunochromatographic test, meaning it produces a visual result (a line) directly. The analytical sensitivity study determined the lowest concentration of virus the device could detect on its own, without human interpretation adding to or detracting from its direct reactivity.

7. The Type of Ground Truth Used

• Type of Ground Truth: For the analytical sensitivity study, the ground truth was quantitated viral concentration (expressed as TCID50/ml or CEID50/ml). This is a laboratory-based quantitative measurement of infectious virus particles, which is highly objective.

8. The Sample Size for the Training Set

• Sample Size (Training Set): The document does not provide a specific sample size for a "training set." This is because the device is a rapid in vitro diagnostic (IVD) immunoassay, not a machine learning or AI-driven device that typically undergoes a training phase. Its performance relies on the biochemical interactions of antigens and antibodies. The "training" in this context would be the initial development and optimization of the assay's reagents and format, for which specific sample sizes are not typically reported in such regulatory summaries.

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

• Ground Truth (Training Set): As mentioned above, there isn't a traditional "training set" in the context of an AI/ML device. For an immunochromatographic assay like the Xpect® Flu A&B, the optimization and development would involve:
  • Selection and characterization of antibodies: Ensuring high specificity and affinity for influenza A and B nucleoproteins.
  • Optimization of reagent concentrations: Determining the ideal amounts of conjugated antibodies, capture antibodies, and other components for optimal signal-to-noise ratio and sensitivity.
  • Optimization of membrane characteristics and flow properties.
  • Ground truth during this development phase would be established by testing against a range of known positive (cultured virus, clinical samples confirmed by cell culture or RT-PCR) and negative samples (other respiratory viruses, healthy controls) to refine the assay parameters. Quantitative viral loads (TCID50/ml or CEID50/ml) would likely have been used as the internal ground truth during these developmental stages to guide improvements in detection limits.

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