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Abstract: In the beginning of 2020, the outbreak of the COVID-19 pandemic led to a crisis in which diagnostic methods for the genome detection of SARS-CoV-2 were urgently needed. Based on the very early publication of the basic principles for a diagnostic test for the genome detection of SARS-CoV-2, the first noncommercial laboratory-developed tests (LDTs) and commercial tests were introduced. As there was considerable uncertainty about the reliability and performance of different tests and different laboratories, INSTAND established external quality assessment (EQA) schemes for the detection of SARS-CoV-2 starting in April 2020. In close partnership in a scientific network, the EQA schemes were enhanced, especially the April, June and November 2020 terms. The enhancement included: (i) immediate provision of suitable virus including variants of concern at the beginning of the pandemic outbreak, (ii) short frequency of EQA schemes, (iii) concentration dependency of the testing and sensitivity check, achieved by using SARS-CoV-2-positive samples from a 10-fold dilution series of the same starting material, (iv) specificity check of the testing, achieved by using SARS-CoV-2-negative samples containing human coronaviruses or MERS CoV, (v) revealed samples for orientation on test performance during an ongoing or at the start of an EQA scheme using a pre-quantified SARS-CoV-2-positive EQA sample with a low viral RNA load of only 1 570 copies/mL assigned by digital PCR (dPCR) in June 2020 and (vi) quantified reference materials based on the experiences of the first two EQA schemes with dPCR-assigned values in copies/mL beginning in November 2020 for self-evaluation of the applied test system. This manuscript summarizes the results of a total of 13 EQA schemes for the detection of SARS-CoV-2 between April 2020 and June 2023 in which a total of 1 413 laboratories from 49 countries participated. The qualitative results for the detection of SARS-CoV-2-positive samples were between 95.8 % and 99.7 % correct positive, excluding extremely low concentration samples. For all SARS-CoV-2-negative EQA samples, the qualitative success rates ranged from 95.1 % to 99.4 % correct negative results. The widely varying values for the cycle threshold (Ct)/crossing point (Cq) reported for the different target genes and test systems were striking. A few laboratories reported quantitative results in copies/mL for several VOCs with an acceptable rate of over 93 % correct positive results in the majority of cases. The description of the enhanced EQA schemes for SARS-CoV-2 detection in terms of timing and scope can serve as a blueprint for the rapid development of a quality assessment of diagnostics for an emerging pathogen.
Keywords: COVID-19 pandemic, SARS-CoV-2, virus genome detection tests, reference materials, external quality assessment, laboratory medicine, epidemiology
Published in DiRROS: 18.06.2025; Views: 496; Downloads: 445
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Abstract: Quantitative PCR (qPCR) is an important tool in pathogen detection. However, the use of different qPCR components, calibration materials and DNA extraction methods reduces comparability between laboratories, which can result in false diagnosis and discrepancies in patient care. The wider establishment of a metrological framework for nucleic acid tests could improve the degree of standardisation of pathogen detection and the quantification methods applied in the clinical context. To achieve this, accurate methods need to be developed and implemented as reference measurement procedures, and to facilitate characterisation of suitable certified reference materials. Digital PCR (dPCR) has already been used for pathogen quantification by analysing nucleic acids. Although dPCR has the potential to provide robust and accurate quantification of nucleic acids, further assessment of its actual performance characteristics is needed before it can be implemented in a metrological framework, and to allow adequate estimation of measurement uncertainties. Here, four laboratories demonstrated reproducibility (expanded measurement uncertainties below 15%) of dPCR for quantification of DNA from human cytomegalovirus, with no calibration to a common reference material. Using whole-virus material and extracted DNA, an intermediate precision (coefficients of variation below 25%) between three consecutive experiments was noted. Furthermore, discrepancies in estimated mean DNA copy number concentrations between laboratories were less than twofold, with DNA extraction as the main source of variability. These data demonstrate that dPCR offers a repeatable and reproducible method for quantification of viral DNA, and due to its satisfactory performance should be considered as candidate for reference methods for implementation in a metrological framework.
Keywords: digital PCR, DNA quantification, inter-laboratory assessment, human cytomegalovirus, virus reference materials
Published in DiRROS: 24.07.2024; Views: 993; Downloads: 621
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Abstract: Well-characterized reference materials support harmonization and accuracy when conducting nucleic acid-based tests (such as qPCR); digital PCR (dPCR) can measure the absolute concentration of a specific nucleic acid sequence in a background of non-target sequences, making it ideal for the characterization of nucleic acid-based reference materials. National Metrology Institutes are increasingly using dPCR to characterize and certify their reference materials, as it offers several advantages over indirect methods, such as UV-spectroscopy. While dPCR is gaining widespread adoption, it requires optimization and has certain limitations and considerations that users should be aware of when characterizing reference materials. This review highlights the technical considerations of dPCR, as well as its role when developing and characterizing nucleic acid-based reference materials.
Keywords: digital PCR, dPCR, reference materials, UV-spectroscopy
Published in DiRROS: 03.06.2024; Views: 1407; Downloads: 550
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