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cobas® BKV is an in vitro nucleic acid amplification test for the quantitation of BKV) DNA in human EDTA plasma and urine stabilized in cobas® PCR media on the cobas® 6800/8800 Systems.

In EDTA plasma, cobas® BKV is intended for use as an aid in the management of BKV in transplant patients. In patients undergoing monitoring of BKV in EDTA plasma, serial DNA measurements can be used to indicate the need for potential treatment changes and to assess viral response to treatment.

In urine stabilized in cobas® PCR Media, cobas® BKV is intended for use as an aid in the management of BKV in transplant patients.

The results from cobas® BKV are intended to be read and analyzed by a qualified licensed healthcare professional in conjunction with clinical signs and symptoms and relevant laboratory findings. Test results must not be the sole basis for patient management decisions .

cobas® BKV is not intended for use as a screening test for blood products or human cells, tissues, and cellular and tissue-based products (HCT/Ps).

cobas® BKV (Figure 1) is based on fully automated sample preparation (nucleic acid extraction and purification) followed by PCR amplification and detection. The cobas® 6800/8800 Systems consist of the sample supply module, the transfer module, the processing module, and the analytic module. Automated data management is performed by the cobas® 6800/8800 software which assigns test results for all tests as either target not detected, BKV DNA detected < LLoQ (lower limit of quantitation), BKV DNA detected > ULoQ (upper limit of quantitation), or a value in the linear range LLoQ < x < ULoQ. Results can be reviewed directly on the system screen, exported, or printed as a report.

Nucleic acid from patient samples and added lambda DNA-OS molecules is simultaneously extracted. In summary, viral nucleic acid is released by addition of proteinase and lysis reagent to the sample. The released nucleic acid binds to the silica surface of the added magnetic glass particles. Unbound substances and impurities, such as denatured protein, cellular debris and potential PCR inhibitors are removed with subsequent wash reagent steps and purified nucleic acid is eluted from the glass particles with elution buffer at elevated temperature.

Selective amplification of target nucleic acid from the sample is achieved by the use of a dual target virus specific approach from highly-conserved regions of the BKV located in the BKV small t-antigen region and the BKV VP2 region. Selective amplification of DNA-QS is achieved by the use of sequence-specific forward and reverse primers which are selected to have no homology with the BKV genome. A thermostable DNA polymerase enzyme is used for amplification. The target and DNA-QS sequences are amplified simultaneously utilizing a universal PCR amplification profile with predefined temperature steps and number of cycles. The master mix includes deoxyuridine triphosphate (dUTP), instead of deoxythimidine triphosphate (dTTP), which is incorporated into the newly synthesized DNA (amplicon).13 Any contaminating amplicon from previous PCR runs is eliminated by the AmpErase enzyme, which is included in the PCR mix, when heated in the first thermal cycling step. However, newly formed amplicons are not eliminated since the AmpErase enzyme is inactivated once exposed to temperatures above 55°C.

The cobas® BKV master mix contains two detection probes specific for BKV target sequences and one for the DNA-QS. The probes are labeled with target-specific fluorescent reporter dyes allowing simultaneous detection of BKV target and DNA-QS in two different target channels.45 The fluorescent signal of the intact probes is suppressed by the quencher dye. During the PCR amplification step, hybridization of the probe to the specific single-stranded DNA templates results in cleavage by the 5'-to-3' nuclease activity of the DNA polymerase resulting in separation of the reporter and quencher dyes and the generation of a fluorescent signal. With each PCR cycle, increasing amounts of cleaved probes are generated and the cumulative signal of the reporter dye is concomitantly increased. Real-time detection and discrimination of PCR products are accomplished by measuring the fluorescence of the released reporter dyes for the viral targets and DNA-OS.

This document describes the acceptance criteria and supporting studies for the cobas® BKV device for the quantitation of BK virus (BKV) DNA in human urine. The information is extracted from a 510(k) summary.

1. Table of Acceptance Criteria and Reported Device Performance

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

The studies focused on analytical performance (Limit of Detection, Linearity, Precision, Specificity, Interference, Cross-contamination) and clinical performance (Reproducibility, Clinical Concordance).

• Limit of Detection (LoD):
  • WHO International Standard: 63 replicates per concentration level (total 7 levels, x3 lots = 1323 replicates).
  • Subgroups/Subtypes Verification: 63 replicates per concentration level for each genotype (total 5 genotypes, 3 levels each, x3 lots = 2835 replicates).
• Linear Range:
  • Main linearity: 36 replicates per panel member (10 panel members, x3 lots = 1080 replicates).
  • Genotype linearity: 12 replicates per level for each genotype (8 panel members each, 5 genotypes, x3 lots = 1440 replicates).
• Lower Limit of Quantitation (LLoQ): Data from the Linearity study at 100, 200, and 300 IU/mL concentrations.
• Precision (Within-Laboratory): 72 replicates for each of 5 dilution levels (x3 lots = 1080 replicates).
• Analytical Specificity: 3 replicates for each of the microorganisms listed in Table 10, both in BKV-negative and BKV-positive urine (number of microorganisms not explicitly totalled, but substantial).
• Interfering Substances: Replicates for each substance in presence and absence of BKV DNA (number of replicates not explicitly stated, but implies multiple for each substance in Table 11).
• Cross Contamination: 240 replicates of BKV-negative matrix samples, 225 replicates of high titer BKV DNA urine samples.
• Clinical Reproducibility: 270 tests per concentration (5 concentrations, total 1350 tests, not including controls).
• Clinical Performance / Concordance: 308 neat urine samples from 84 transplant subjects (for concordance analysis). 61 negative samples for NPA. 153 BKV positive urine samples from 55 transplant subjects (for correlation analysis).

Data Provenance: The document implies that the non-clinical studies were conducted internally or at authorized labs. The clinical performance evaluation was conducted at 3 testing sites, suggesting multi-site prospective data collection. The data samples were human EDTA plasma and urine. The origin of the samples (country) is not explicitly stated in the provided text. The clinical study used a retrospective cohort of samples from transplant patients.

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

The ground truth for the analytical studies was established based on known concentrations of BKV international standards or armored DNA. For clinical performance, the comparator was a "well-established laboratory developed nucleic acid test (LDT)".

The document does not mention the use of experts to establish ground truth for the test sets in the typical sense of human readers for image-based diagnostics. The "ground truth" for this diagnostic device study is based on the highly controlled properties of the spiked samples (known concentrations, genotypes) for analytical performance, and the results from a comparator LDT for clinical concordance.

4. Adjudication Method for the Test Set

Not applicable in the context of this in vitro diagnostic device, as the "ground truth" is based on quantitative measurements and known concentrations, not subjective expert assessment requiring adjudication.

5. If a Multi-Reader Multi-Case (MRMC) Comparative Effectiveness Study was done, If so, what was the effect size of how much human readers improve with AI vs without AI assistance

Not applicable. This device is an in vitro nucleic acid amplification test (NAAT) for quantitative measurement of BKV DNA. It is not an AI-assisted diagnostic device requiring human reader input or interpretation in the way an imaging diagnostic device would. Therefore, an MRMC comparative effectiveness study involving human readers and AI assistance is not relevant to this submission.

6. If a Standalone (i.e. algorithm only without human-in-the-loop performance) was done

Yes, the studies described are for the standalone performance of the cobas® BKV system, which is an automated molecular diagnostic assay. The system performs "fully automated sample preparation (nucleic acid extraction and purification) followed by PCR amplification and detection." The results are "assigned... by the cobas® 6800/8800 software." While "results are intended to be read and analyzed by a qualified licensed healthcare professional in conjunction with clinical signs and symptoms and relevant laboratory findings," the primary performance metrics are based on the direct output of the automated system.

7. The Type of Ground Truth Used (expert consensus, pathology, outcomes data, etc.)

• Analytical Studies: The ground truth for analytical studies (LoD, Linearity, Precision, Specificity, Interference) was established using known concentrations of BKV DNA, including the WHO International Standard (NIBSC 14/212), BKV armored DNA, and clinical specimens diluted to specific concentrations. Samples were spiked into BKV-negative urine.
• Clinical Studies: For clinical concordance, the ground truth was based on the results from a "well-established laboratory developed nucleic acid test (LDT) (comparator BKV LDT)" on actual clinical urine samples. DNA sequencing was also used in some cases to confirm BKV presence in discordant results.

8. The Sample Size for the Training Set

The document describes performance evaluation studies (validation and verification) rather than a machine learning model development process that typically involves distinct training and test sets.
Therefore, a separate "training set" sample size for a machine learning algorithm is not applicable in the context of this in vitro diagnostic device, which is based on established molecular biological techniques (PCR).

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

As this is not an AI/ML-based device with a "training set," this question is not applicable. The ground truth for the evaluation of the device was established through known concentrations of viral standards and comparison to a comparator LDT, as described in point 7.

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