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Combination of rRT-PCR and Anti-Nucleocapsid/Anti-Spike Antibodies to Characterize Specimens with Very Low Viral SARs-CoV-2 Load: A Real-Life Experience

Published: 10 June 2021
 by  MDPI AG
Microorganisms , Volume 9; doi:10.3390/microorganisms9061263

Abstract: The objective of the present study was to evaluate the true positivity among people, whose results of initial testing of nasopharyngeal swabs (NPS) showed a very low viral load of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Seventy-seven people detected with low viral loads of SARs-CoV-2 in nasopharyngeal samples (Ct ≥ 35) were enrolled in the study. For this purpose, a second NPS was collected for rRT-PCR (real-time reverse transcription polymerase chain reaction) combined with a pair of serum samples for detection of anti-nucleocapsid (anti-N) and anti-spike (anti-S) antibodies. In 8 people, subsequent examinations indicated an increase in viral loads, thereafter, followed by an increase of anti-N and anti-S antibodies, findings compatible with an early stage of COVID-19 infection. In 9 people, who already had increased anti-N antibodies, subsequent examination showed a decrease or absence of viral load and an increase in antibodies, indicative of a late stage of COVID-19 infection. In 60 people, subsequent examination showed absence of infection (as indicated by absence of viral load and antibodies). We propose that the combination of a second NPS and one serum-specimen, both taken three days after the first NPS, helps significantly to avoid false-positive results.
Keywords: anti-nucleocapsid antibodies / anti-spike antibodies / COVID-19 / diagnosis / false positive / Greece / low viral load / N gene

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