John Lewis Winkler
Authors: Geraldine Winkler
Journal: BMJ (2015)

<h3>Abstract</h3> <h3>Background</h3> The current standard for COVID-19 diagnosis, RT-qPCR, has important drawbacks for its use as a tool for epidemiological control, including the need of laboratory-processing, high cost, and long turnaround from sampling to results release. Antigen-based rapid diagnostic tests (Ag-RDT) provide a promising alternative for this purpose. <h3>Methods</h3> We assessed the analytical and clinical performance of the Ag-RDT Panbio COVID-19 Ag Test (Abbott), using RT-qPCR as a reference test. The clinical performance was assessed using nasopharyngeal swabs, collected in routine practice for case confirmation and contact tracing, and nasal mid-turbinate swabs, collected in preventive screenings of asymptomatic individuals. Fresh samples were analysed by RT-q-PCR, stored at -80 °C, and analysed using the Ag-RDT according to the manufacturer instructions. <h3>Findings</h3> The Ag-RDT had a limit of detection of 6·5×10<sup>5</sup> copies/reaction. The clinical performance was assessed on 1,406 frozen swabs with a PCR result available: 951 (67·7%) positive and 455 (32·4%) negative. The Ag-RDT identified the presence of SARS-CoV-2 in 872 of 951 PCR-positive samples (91·7%; 95% CI 89·8-93·4 and ruled out its presence in 450 of 455 PCR-negative samples (specificity 98·9%; 95% CI 97·5– 99·6). Sensitivity increased in samples with lower Ct values (Ct &lt;25, 98·2%; Ct&lt;30, 94·9%) and was higher among symptomatic cases (92·6%) and their contacts (94·2%) than among asymptomatic individuals (79·5%). In the setting of asymptomatic screening, sensitivity also increased with lower Ct values (Ct &lt;25, 100%; Ct&lt;30, 98·6%). Assuming a pre-test probability of 5%, the negative and positive predictive values were 99·6% (99·5 – 99·6) and 81·5% (65·0 – 93·2), respectively. <h3>Interpretation</h3> The Panbio COVID-19 Ag-RDT has high sensitivity for detecting the presence of SARS-CoV-2 in nasal or nasopharyngeal swabs of both, symptomatic and asymptomatic individuals. The diagnostic performance of the test is particularly good in samples with viral loads associated with high risk of viral transmission (Ct &lt;25), which show high positive and negative predictive values even when assuming a prevalence as low as 5%. <h3>Funding</h3> Blueberry diagnostics, Fundació Institut d’Investigació en Ciències de la Salut Germans Trias i Pujol, and #YoMeCorono.org crowfunding campaing. <h3>Research in context</h3> <h3>Evidence before this study</h3> On October 6, 2020, we searched PubMed for articles containing “Antigen”, “test”, “SARS-CoV-2”, “COVID-19” and “performance” in either the title or the abstract. We found five studies that showed the accuracy of point-of-care tests in identifying SARS-CoV-2 antigens for confirmation of clinically suspected COVID-19. We found high variability in the diagnostic accuracy of Ag-RDT. Most tests showed high specificity (i.e., 99% or higher), whereas sensitivity ranged from 11% to 92%; only one test reported sensitivity higher than 60%. We found no studies investigating the diagnostic accuracy of the <i>Panbio COVID-19 Ag Test</i>. We found no studies that assessed the performance of Ag-RDT for population-level screening of asymptomatic individuals. <h3>Added value of this study</h3> Our analysis provides information regarding the diagnostic accuracy of the <i>Panbio COVID-19 Ag Test</i> when tested on 1,406 frozen samples of nasopharyngeal and nasal swabs collected in routine practice for diagnostic confirmation of symptomatic individuals with suspected COVID-19 or contacts exposed to a positive case, and preventive screenings of unexposed asymptomatic individuals. Compared with RT-qPCR as reference test, the Ag-RDT showed a sensitivity and specificity of 91·7% and 98·9%. Test sensitivity increased in samples with viral load associated with high risk of transmission (Ct &lt;25), reaching more than 98%, regardless of the presence of symptoms. <h3>Implications of all the available evidence</h3> Available evidence show variability in the diagnostic performance of marketed Ag-RDT. Our results provide substantial evidence that the point-of-care <i>Panbio COVID-19 Ag Test</i> can accurately identify SARS-CoV-2 antigens in people with suspected clinical COVID-19 as well as in asymptomatic people with high viral load and therefore, associated with higher risk of transmission. This finding represents a potentially useful advance for mass screening of asymptomatic people at the point-of-care.