SARS-CoV-2 viral load monitoring by extraction-free testing of saliva

Abstract

Real-time quantitative reverse transcriptase polymerase chain reaction (RT-qPCR) remains the foundation of SARS-CoV-2 testing due to its accessibility, scalability, and superior assay performance. Variability in specimens and methods prevent standardization of RT-qPCR assays and reliable quantitative reporting to assess viral load. We developed an extraction-free RT-qPCR assay for detection of SARS-CoV-2 in saliva and monitored viral load until convalescence in COVID-19 patients. Comparison of 231 matched anterior nares swab and saliva specimens demonstrated that extraction-free testing of saliva has equivalent analytical and clinical assay performance compared to testing of RNA extracts from either anterior nares or saliva specimens. Analysis of specimen pairs revealed higher viral loads in the nasal cavity compared to the oral cavity, although this difference did not impact clinical sensitivity for COVID-19. Extraction-free testing of a combination specimen consisting of both nasal swab and saliva is also demonstrated. Assessment of viral load by RT-qPCR and parallel digital droplet PCR (ddPCR) revealed that cycle threshold (Ct) values less than approximately 30 correlated well with viral load, whereas Ct values greater than 30 correspond to low viral loads <10 copies/uL. Therefore, extraction-free saliva testing maximizes testing efficiency without compromising assay performance and approximates viral loads >10 copies/uL. This technology can facilitate high-throughput laboratory testing for SARS-CoV-2, monitor viral load in individual patients, and assess efficacy of therapies for COVID-19.