Evaluation of SARS-CoV-2 rapid antigen tests using a saliva sample

In a recent study published on medRxiv* server, researchers evaluated a rapid antigen test (RAT) that used saliva samples to detect the presence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).

Study: Comparison of COVID-19 rapid antigen test (oral fluid) and real-time RT-PCR in the laboratory diagnosis of SARS-CoV-2 infection. Image Credit: Dan Race/Shutterstock

Background

The World Health Organization (WHO) recommends a real-time reverse transcription-polymerase chain (RT-PCR) test that uses nasal/nasopharyngeal specimens as the gold standard for detecting SARS-CoV-2. Qualified personnel perform these tests and require a sophisticated lab setup, and their sample preparation and run time is approximately two hours.

However, due to the limited availability of these tests in many places, self-use RATs are needed. RATs also potentially reduce the burden on diagnostic laboratories, which has multiplied during the coronavirus disease 2019 (COVID-19) pandemic. Currently, two types of SARS-CoV-2 RATs are available; one that uses nasal swabs and the other that uses oral fluid. The latter is gaining interest because it is easier to collect saliva.

About the study

In the present study, researchers evaluated a RAT developed by Hangzhou Alltest Biotech that detects SARS-CoV-2 nucleocapsid (NC) protein in human oral fluid samples. They tested this device on 500 self-collected saliva samples from patients by spitting them into sterile tubes. The researchers ensured that these patients had not consumed any food, drink, chewing gum or tobacco products for at least 10 minutes before the sample was taken. They performed the test according to the instruction manual and got the results in about 15 minutes.

For each nucleic acid isolation step, researchers used negative and positive controls, deionized water, and an independent run control (IRC). According to the manufacturer’s instruction manual, saliva samples with a cycle threshold (CJ) for one or both SARS-CoV-2 NC genes and open reading frame 1ab (ORF1ab) were considered positive.

Study results

The study test met the WHO sensitivity criteria in samples containing CJ values ​​above 30 or high viral loads. It had a sensitivity and specificity of 90.1% and 99.3%, respectively, and an accuracy of 97%. For SARS-CoV-2 positive saliva specimens with a CJ below 30, its sensitivity and accuracy drop to 83.8% and 95.9% respectively. A higher CJ indicates a lower viral load in the sample, which may be difficult for the RAT to detect. Nevertheless, a negative RAT result does not rule out SARS-CoV-2 infection and requires further verification by RT-PCR.

Additionally, the RAT required little or no equipment beyond the hardware provided in its kit. It could be transported easily to one place and done in less than an hour. Sample preparation took less than five minutes and results were typically available in fifteen minutes.

conclusion

The researchers evaluated a RAT using saliva samples in the present study, which met the high standards set by the WHO and was non-invasive, affordable and easy to use. It showed an overall sensitivity and specificity of 66.7% and 100% in samples with a CJ value less than 37. It also easily distinguished SARS-CoV-2 infected individuals with high and low viral loads. In support of the findings of this study, another study by Basso et al. showed that saliva is an excellent alternative to a nasopharyngeal swab in early SARS-CoV-2 infections.

*Important Notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be considered conclusive, guide clinical practice/health-related behaviors, or treated as established information.

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