A negative SARS-CoV-2 RAT result with a Roche Cobas RAT indicates a non-contagious individual, even if PCR positive.

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• Harald H Kessler, Florian Prüller, Melina Hardt, Evelyn Stelzl, Esther Föderl-Höbenreich, Sabine Pailer, Andreas Lueger, Philipp Kreuzer, Kurt Zatloukal, and Markus Herrmann.
• Molecular Diagnostics Laboratory, Diagnostic and Research Institute of Hygiene,Microbiology, and Environmental Medicine, Medical University of Graz, Graz, Austria.
• Clin Chem Lab Med. 2022 Apr 26; 60 (5): 778-785.

ObjectivesRapid antigen tests (RAT) can provide valuable information on the presence or absence SARS-CoV-2 within 15 min without the need of a laboratory. The analytical and diagnostic characteristics of available RATs has led to the question whether they can safely distinguish between infectious and non-infectious patients in an acute care setting.MethodsThree nasopharyngeal swabs for the analysis by RAT, reverse transcriptase real time polymerase chain reaction (RT-qPCR), and a cell culture based infection assay were collected from 67 patients that presented to the emergency department of the University Hospital of Graz (Austria). The first swab was used for on-site RAT testing in the emergency department using the Roche SARS-CoV-2 RAT. The second swab was sent to the central laboratory of the hospital for RT-qPCR with two independent methods (Cepheid Xpert® Xpress SARS-CoV-2 assay and Roche Cobas SARS-CoV-2 Test) and repeat RAT testing using the same commercial test. With the third swab a cell culture-based infection assay was performed.ResultsThe RATs performed from independent samples showed substantial agreement (Cohen's-kappa: 0.73, p<0.001). All patients with a positive RAT had positive RT-qPCR with cycle threshold (ct) values <25. Fifteen out of 55 RAT-negative samples were RT-qPCR positive with ct values between 25 and 40. The inoculation of cell cultures with RT-qPCR negative swabs and RT-qPCR positive swabs with ct values >25 did not induce cytopathic effects that were related to SARS-CoV-2. The infection assays from four RAT-negative patients showed cytopathic effects that were induced by other pathogens.ConclusionsThe SARS-CoV-2 RAT from Roche Diagnostics is a valuable tool for managing symptomatic patients. RAT-negative patients may be regarded as non-contagious.© 2022 Walter de Gruyter GmbH, Berlin/Boston.

This article appears in the collection: Does a COVID RAT-negative result mean non-infectious?.

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