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Meals and Room Temperature Storage do not Significantly Affect Feasibility of Direct RT-PCR Tests for SARS-CoV-2 Using Saliva by Koji Tsuchiya, Yoshie Hosaka, Toshihiro Takahashi, Masayoshi Chonan, Yuko Makita, Isana Katayama, Tomohiko Ai, Mayumi Idei, Yuki Horiuchi, Kotoko Yamatani, Mitsuru Wakita, Shigeki Misawa, Atsushi Okuzawa, Takashi Miida, Toshio Naito, Kazuhisa Takahashi, Yoko Tabe

Background: Rapid detection of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) using saliva samples has emerged as a preferred technique since sample collection is easy and noninvasive. In addition, several commercial high-throughput PCR kits that do not require RNA extraction/purification have been developed and are now available for testing saliva samples. However, an optimal protocol for SARS-CoV-2 RT-PCR testing of saliva samples using the RNA extraction/purification-free kits has not yet been established. The aim of this study was to establish optimal preanalytical conditions, including saliva sample collection, storage, and dilution for RNA extraction/purification-free RT-PCR (direct RT-PCR).
Methods: Patients suspected with COVID-19 from March 02 to August 31, 2020, were enrolled in this study. A total of 248 samples, including 43 nasopharyngeal swabs and 205 saliva samples, were collected from 66 patients (37 outpatients and 29 inpatients) and tested using the 2019 Novel Coronavirus Detection Kit (nCoV-DK, Shimadzu Corporation, Kyoto, Japan).
Results: The detection results obtained using nasopharyngeal swabs and saliva samples matched 100%. The sampling time, i.e., either awakening time or post-breakfast, had no significant effect on the viral load of the saliva samples. Although saliva samples are routinely diluted to reduce viscosity, we observed that dilution negatively affected PCR sensitivity. Saliva samples could be stored at room temperature (25°C) for 24 hours or at 4°C for up to 48 hours.
Conclusions: This study demonstrated the appropriate conditions of saliva sample collection, processing, and storage, and indicated that the nCoV-DK is applicable to saliva samples, making the diagnosis method simple and safe.

DOI: 10.7754/Clin.Lab.2021.210451