Validation of a Saliva-Based Test for the Molecular Diagnosis of SARS-CoV-2 Infection

BackgroundSince the beginning of the pandemic, clinicians and researchers have been searching for alternative tests to improve screening and diagnosis of SARS-CoV-2 infection (Y. Yang et al., medRxiv 2020; W. Wang et al., 2020.3786; A Senok et al., Infect Drug Resist 2020). Currently, the gold standard for virus identification is the nasopharyngeal (NP) swab (N. Sethuraman et al., JAMA 2020; A.J. Jamal et al Clinical Infect Disease 2021). Saliva samples, however, offer clear practical and logistical advantages (K.K.W To et al, Clinical Microb and Infect; A.L. Wylle et al. N Engl J Med 2020; N. Matic et al, Eur J Clin 2021) but due to lack of collection, transport, and storage solutions, high-throughput saliva-based laboratory tests are difficult to scale up as a screening or diagnostic tool (D. Esser et al., Biomark Insights 2008; E. Kaufman et al., Crit Rev Oral Biol Med2002). With this study, we aimed to validate an intra-laboratory molecular detection method for SARS-CoV-2 on saliva samples collected in a new storage saline solution, comparing the results to NP swabs to determine the difference in sensitivity between the two tests.MethodsIn this study, 156 patients (cases) and 1005 asymptomatic subjects (controls) were enrolled and tested simultaneously for the detection of the SARS-CoV-2 viral genome by RT-PCR on both NP swab and saliva samples. Saliva samples were collected in a preservative and inhibiting saline solution (Biofarma Srl). Internal method validation was performed to standardize the entire workflow for saliva samples.ResultsThe identification of SARS-CoV-2 conducted on saliva samples showed a clinical sensitivity of 95.1% and specificity of 97.8% compared to NP swabs. The positive predictive value (PPV) was 81% while the negative predictive value (NPV) was 99.5%. Test concordance was 97.6% (Cohen’s Kappa=0.86; 95% CI 0.81-0.91). The LoD of the test was 5 viral copies for both samples.ConclusionsRT-PCR assays conducted on a stored saliva sample achieved similar performance to those on NP swabs and this may provide a very effective tool for population screening and diagnosis. Collection of saliva in a stabilizing solution makes the test more convenient and widely available; furthermore, the denaturing properties of the solution reduce the infective risks belonging to sample manipulation.