A combined oro-nasopharyngeal swab is more sensitive than mouthwash in detecting SARS-CoV-2 by a high-throughput PCR assay

Michel, W.; Farber, J.; Dilas, M.; Tammer, I.; Baar, J.; Kaasch, A. J.

Objectives: The optimal diagnostic specimen to detect SARS-CoV-2 by PCR in the upper respiratory tract is unclear. Mouthwash fluid has been reported as an alternative to nasopharyngeal and oropharyngeal swabs. We compared mouthwash fluid with a combined oro-nasopharyngeal swab regarding test performance. Methods: We tested asymptomatic persons with a previous diagnosis of COVID-19 and their household contacts. First, a mouthwash (gargling for at least 5 sec) with sterile water was performed. Then, with a single flocked swab the back of the throat and subsequently the nasopharynx were sampled. Samples were inactivated and analysed on a Roche cobas 6800 system with the Roche SARS-CoV-2 test. Results: Of 76 persons, 39 (51%) tested positive for SARS-CoV-2 by oro-nasopharyngeal swab. Mouthwash detected 13 (17%) of these infections but did not detect any additional infection. Samples that were positive in both tests, had lower cycle threshold (Ct)-values for oro-nasopharyngeal samples, indicating a higher virus concentration, compared to samples only positive in oro-nasopharyngeal swabs. Conclusions: Mouthwash is not as sensitive as combined oro-nasopharyngeal swab in detecting upper respiratory tract infection.