Clinical Validation of the Curetis SARS-CoV-2 Kit with PULB using dry swabs

TECHNICAL NOTE

Clinical Validation of the Curetis SARS-CoV-2 Kit with PULB using dry swabs

Introduction

Millions of SARS-CoV-2 tests have been and continue to be performed during the global coronavirus disease (COVID-19) pandemic. Standard RT PCR systems for the detection of SARS-CoV-2 require RNA isolation (extraction) kits, further stressing the already burdened supply chains and manufacturers worldwide, limiting the capacity of rapid diagnosis of COVID-19. The Curetis SARS-CoV-2 Kit with PULB offers the possibility of performing a PCR without the necessity of RNA isolation. The kit contains a PCR-compatible universal lysis buffer (PULB) that allows to use dry swab collection as well as swabs collected in PCR-compatible viral transport medium treated with the unique PULB provided in the kit.

Study Overview

Performance comparison of the Curetis SARS-CoV-2 Kit with PULB against standard SARS-CoV-2 detection method was evaluated by an independent external study site, in the microbiology laboratory of the University Clinical Center of Republic of Srpska, Banja Luka, RS, Bosnia and Herzegovina.

Reference Method (standard SARS-CoV-2 PCR with extraction):

Specimens used included nasopharyngeal / oropharyngeal swabs collected in viral transport medium, as well as sputum and BAL. RNA was isolated from specimens using NuActor, an automated nucleic acid isolation system with viral RNA nucleic acid isolation reagents (Boditech Med Inc.). Five microliters (5 µL) of isolated RNA was then used as template for SARS-CoV-2 detection using GeneFinder^TM COVID-19 Plus RealAmp Kit (OSANG Healthcare Co).

Curetis SARS-CoV-2 Kit with PULB:

Dry swab collection of the nasopharyngeal / oropharyngeal was obtained in parallel with the standard collection method from the same study subjects. The dry swab collections were treated with PULB following the manufacturer’s instructions. In short, dry swabs were put in 200 µL of PULB, swirled and squeezed out. The obtained buffer was then incubated for 5 min at 95 °C and 1 µL was taken as PCR template for SARS-CoV-2 RT PCR performed on a Quant Studio 5 RT PCR system.

For the comparison, Ct values of 35 and lower were defined as a positive result for SARS-CoV-2 detection.

Results

A total of 40 samples were tested in parallel using the Curetis SARS-CoV-2 Kit with PULB and GeneFinder™ COVID-19 Plus RealAmp Kit as the reference method. Thirty (30) samples were positive and ten (10) samples were negative using GeneFinder COVID-19 Plus RealAmp Kit. Samples with undefined GeneFinder results were not included in the test. Of the 30 positives samples, 27 were positive using the Curetis SARS-CoV-2 Kit with PULB, two were flagged as “undefined” by the study site and one was negative (Ct with GeneFinder COVID-19 Plus RealAmp Kit was 35 for those 3 cases). For the two cases with weak positive PCR results flagged as “undefined” by the clinical site, the Ct values determined by the Curetis SARS-CoV-2 Kit with PULB were 37 and 39, respectively, and these ambiguous cases were not included in further analysis.

All 10 samples that were negative with GeneFinder COVID-19 Plus RealAmp Kit were also negative using the Curetis SARS-CoV-2 Kit with PULB. For the 27 positive detections using the Curetis SARS-CoV-2 Kit with PULB, Ct values were either the same for both methods or in most cases differed only by ± 1-2 Cts. Detailed distribution of Ct values are shown in Figure 1.

Figure 1. Distribution of Ct values of the Curetis SARS-CoV-2 kit with PULB vs. Ct values of the study site’s standard method (GeneFinder)

Resulting performance data are listed in the table below:

In conclusion, the results showed that the performance of the Curetis SARS-CoV-2 Kit with PULB using dry swabs is comparable with the performance of GeneFinder^TM COVID-19 Plus RealAmp Kit using isolated RNA. Furthermore, time and money can be saved by using the Curetis SARS-CoV-2 Kit with PULB which eliminates the need for standard RNA isolation step otherwise performed for each sample.

The Curetis SARS-CoV-2 Kit with PULB is CE IVD marked but it has not been authorized for emergency use by the U.S. FDA.