ABSTRACT
The Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), etiological agent of the coronavirus disease 2019 (COVID-19), is currently detected by reverse transcription followed by quantitative polymerase chain reaction (RT-qPCR) of its viral RNA genome. Within the available alternatives, One-Step procedures are preferred since they are fast and significantly decrease preanalytical errors, minimizing the risk of diagnostic errors. Increasing the testing capacity and tracing contacts are essential steps to control the pandemic. However, high-cost commercial reagents subject to shortage and poor scalability have hindered the use of these technologies and their adoption for a wide population-scale testing, being even more critical in developing countries. In the current context, open-source initiatives have promoted global collaboration to promote accessible solutions for rapid local deployment. As a result, open protocols are being developed for the local production of SARS-CoV-2 diagnostics. This work aimed to produce an open-source system for SARS-CoV-2 diagnostic tests in RNA clinical samples. We provide guidelines for standardizing an open One-Step RT-qPCR master mix using recombinant M-MLV reverse transcriptase together with either Pfu-Sso7d or Taq DNA polymerase. Both were tested on synthetic RNA and clinical samples, observing a good correlation when compared to commercial RT-qPCR kits. Nevertheless, the best results were obtained using M-MLV RT combined with Taq DNA polymerase in a probe-based RT-qPCR assay, allowing successful discrimination between positive and negative samples with accuracies comparable to a CDC-recommended commercial kit. Here, we demonstrate that these open RT-qPCR systems can be successfully used to identify SARS-CoV-2 in clinical samples and potentially be implemented in any molecular diagnostic laboratory.
ABSTRACT
RT-LAMP (reverse transcription - Loop-mediated isothermal amplification) has gained popularity for the detection of SARS-CoV-2. The high specificity, sensitivity, simple protocols and potential to deliver results without the use of expensive equipment has made it an attractive alternative to RT-PCR. However, the high cost per reaction, the centralized manufacturing of required reagents and their distribution under cold chain shipping limits RT-LAMPs applicability in low-income settings. The preparation of assays using homebrew enzymes and buffers has emerged worldwide as a response to these limitations and potential shortages. Here, we describe the production of Moloney murine leukemia virus (M-MLV) Reverse Transcriptase and BstLF DNA polymerase for the local implementation of RT-LAMP reactions at low cost. These reagents compared favorably to commercial kits and optimum concentrations were defined in order to reduce time to threshold, increase ON/OFF range and minimize enzyme quantities per reaction. As a validation, we tested the performance of these reagents in the detection of SARS-CoV-2 from RNA extracted from clinical nasopharyngeal samples, obtaining high agreement between RT-LAMP and RT-PCR clinical results. The in-house preparation of these reactions results in an order of magnitude reduction in costs, and thus we provide protocols and DNA to enable the replication of these tests at other locations. These results contribute to the global effort of developing open and low cost diagnostics that enable technological autonomy and distributed capacities in viral surveillance.