Saliva as sensitive as but cheaper than nasopharyngeal swabs for COVID-19 testing

Saliva sampling yields similar sensitivity but lower costs than nasopharyngeal swabs for detecting SARS-CoV-2, the causative agent of the coronavirus disease 2019 (COVID-19), a study has shown.

“Given these findings, plus the advantages of reduced invasiveness, reduced need for trained health care professionals, lower risk for occupational exposure, and reduced need for specialized supplies, we suggest that saliva sampling should replace nasopharyngeal swabs in most populations being tested for SARS-CoV-2,” the researchers said.

The databases of Embase, Medline, medRxiv, and bioRxiv were searched for studies including at least five paired nasopharyngeal swab and saliva samples and reporting diagnostic accuracy for SARS-CoV-2 detection from 1 January to 1 November 2020. Cost inputs were from nationally representative sources in Canada and were converted to US dollars.

The researchers extracted data independently using standardized forms and assessed study quality using the Quality Assessment of Diagnostic Accuracy Studies 2.

Thirty-seven studies with 7,332 paired samples met the eligibility criteria. Saliva sampling showed 3.4 percentage points lower (95 percent confidence interval [CI], 9.9 percentage points lower to 3.1 percentage points higher) sensitivity than that of nasopharyngeal swabs relative to a reference standard of a positive result on either sample. [Ann Intern Med 2021;doi:10.7326/M20-6569]

Among individuals with previously confirmed SARS-CoV-2 infection, the sensitivity of saliva was 1.5 percentage points higher (95 percent CI, 7.3 percentage points lower to 10.3 percentage points higher) than that of nasopharyngeal swabs.

Among those without a previous diagnosis, saliva sensitivity was 7.9 percentage points lower (95 percent CI, 14.7 percentage points less to 0.8 percentage point more). For every 100,000 individuals with a SARS-CoV-2 prevalence of 1 percent tested in this subgroup, nasopharyngeal swabs would detect 79 more (95 percent uncertainty interval, 5 fewer to 166 more) persons with SARS-CoV-2 than saliva, but with an incremental cost of $8,093 per additional infection detected.

“Saliva sampling is an immediate way to expand testing access, while freeing up much-needed health care resources. [It] has already launched in some jurisdictions, and a laboratory protocol has received emergency use authorization from the US Food and Drug Administration,” the researchers said. [https://splunk-public.machinedata.illinois.edu/en-US/app/uofi_shield_public_APP/home; www.fda.gov/news-events/press-announcements/coronavirus-covid-19-update-fda-issues-emergency-use-authorization-yale-school-public-health]

However, analytic methods had to be validated by laboratories analysing saliva. This can be implemented more quickly than approving, producing, and distributing new tests, such as those intended to be used daily or at point of care. [medRxiv 2020;doi:10.1101/2020.06.22.20136309]

Previous systematic reviews and meta-analyses compared saliva sampling with nasopharyngeal swabs, but these were done earlier in the pandemic and were limited by few studies and participant populations. The current meta-analysis improved on this literature with more diverse populations, settings, and saliva collection methods, according to the researchers. [Front Med (Lausanne) 2020;7:465; medRxiv 2020;doi:10.1101/2020.08.05.20168716]

The current study, however, was limited by the imperfect reference standard and varied procedures for collecting saliva samples, they noted.