Discrepancy of SARS-CoV-2 PCR results due to the sample collection sites and possible improper sampling

Abstract

Polymerase chain reaction (PCR) testing for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is necessary for confirming a diagnosis of Coronavirus disease 2019 (COVID-19). Here we present a COVID-19 case of an elderly woman whose SARS-CoV-2 PCR tests showed false negative repeatedly by evaluating with different sampling sites and procedures. Nasopharyngeal swabs, suctioned sputum, and tongue swabs were collected for SARS-CoV-2-PCR. As for tongue swabs, we compared between two different sample conditions; one obtained with dry condition and the other obtained with moistened condition inside the oral cavity. SARS-CoV-2-PCR showed positive for an extended period with suctioned sputum samples compared with nasopharyngeal swabs and tongue swabs. No SARS-CoV-2 from a nasopharyngeal swab sample obtained on day 46 after symptoms onset was isolated despite high viral load (183740.5 copies/5μL). An adequate production of neutralizing antibody in a serum sample on day 46 was also confirmed. The number of RNA copies of the tongue swab samples was higher with moistened condition than with dry condition. The present case suggests that the difference of sampling site or sample condition can affect PCR results. High loads viral RNA detection does not always correlate with infectivity.

Keywords: COVID-19; False-negative; Infectivity; PCR; SARS-CoV-2.

Fig. 1

Chest computed tomography scan performed on the day of COVID-19 diagnosis. Slight ground-glass opacity in the right lower lung lobe.

Fig. 2

Tongue wiping for PCR examination with dry condition (A), and wet condition after moistened with saline (B).

Fig. 3

Clinical course of the present case. PCR, polymerase chain reaction; RNA, ribonucleic acid; ABPC/SBT, ampicillin-sulbactam.