In a mass-screening study of 1,924 asymptomatic persons, self-collected saliva samples had sensitivity (92%) and specificity (over 99.9%) comparable to nasopharyngeal specimens — sensitivity 86% and specificity over 99.9%. The true concordance probability between the nasopharyngeal and saliva tests was estimated at 0.998.

In a mass-screening study of 1,924 asymptomatic persons using nasopharyngeal and self-collected saliva specimens found the sensitivity of molecular tests — RT-PCR and RT-LAMP — was higher in the case of saliva (92%) samples than nasopharyngeal specimens (86%). The specificity was greater than 99.9% in both specimens. The true concordance probability between the nasopharyngeal and saliva tests was estimated at 0.998.

“Both nasopharyngeal and saliva specimens had high sensitivity and specificity. Self-collected saliva is a valuable specimen to detect SARS-CoV-2 in mass screening of asymptomatic persons,” write Isao Yokota from Hokkaido University Graduate School of Medicine, Sapporo, Japan, who led the study. The results are posted on medRxiv, a preprint server. Preprints are yet to be peer-reviewed and published in scientific journals.

Viral load in saliva samples

While previous studies have found mixed results regarding viral load in saliva samples collected from people with confirmed COVID-19 disease, the present study found that viral load was comparable in both types of specimens — nasopharyngeal and saliva. “Our results clearly show the viral loads to be equivalent between nasopharyngeal and saliva in asymptomatic individuals and both specimens may be useful in detecting viral RNA.”

The two cohorts studied include asymptomatic persons who have been in close contact with clinically confirmed COVID-19 patients and asymptomatic travellers arriving at Tokyo and Kansai international airports. The people in the close contact cohort were enrolled between June 12 and July 7 at several centres in Japan, while travellers at airports were enrolled from June 12-23.

All the 1,924 people enrolled in the study provided both nasopharyngeal and self-collected saliva specimens for testing.

Sensitivity and specificity

In the close contact cohort, the virus was detected in 41 nasopharyngeal samples and in 44 saliva samples, of which 38 individuals had both samples test positive. In all, 114 persons were negative in both tests, which resulted in 152 of 161 matches. In the 1,763 airport traveller cohort, viral RNA was detected in five and four nasopharyngeal and saliva samples, respectively.

The sensitivity of nasopharyngeal and saliva were 86% and 92%, respectively and the specificity of nasopharyngeal and saliva were over 99.9% in both types of samples.

Advantage saliva

If rapid detection of asymptomatic people is critical in cutting the transmission chain, mass screening using self-collected saliva can be an added advantage. While nasopharyngeal sample collection is invasive, requires trained healthcare workers and has an inherent risk of exposing health-care workers to the virus at the time of sample collection, self-collected saliva samples avoid all these challenges. As a result, self-collected saliva samples are likely to increase testing compliance.

Further, saliva samples can be collected from people during mass screening without any delay. “Obtaining saliva is simply more conducive to simultaneous mass screening of large number of individuals, in settings such as social and sporting events,” they write.