Testing Saliva as Reliable as Nasal Swab and Detects 93% of COVID-19 Infections, Finds Study

A real-world trial has demonstrated that COVID-19 tests of self-collected saliva provided comparable results to tests performed by trained healthcare professionals using Nasopharyngeal (NP) swabs.

Based on their analysis, the researchers from Beth Israel Deaconess Medical Center (Boston, MA, USA) concluded that saliva tests detect 93% of COVID-19 infections in an outpatient setting. The findings could help alleviate the testing bottlenecks that have constrained COVID-19 testing since early in the pandemic.

As the highly transmissible Delta variant of the virus that causes COVID-19 surges in the US and around the world, rapid and accessible COVID-19 testing and increased vaccination are key to managing the virus’ spread. NP swabs - long, medical-grade probes used to collect samples from deep in patients’ noses and throats - are the gold standard for COVID-19 diagnostic testing. However, memory of shortages of both the specialized NP swabs during the early days of the pandemic as well as pressure on the trained medical personnel needed to perform the tests point to the need for simpler COVID-19 testing.

For their study, the researchers obtained two samples – one via NP swab, the other saliva – from 385 patients presenting for COVID-19 testing. Patients waiting in line for NP testing were given a sterile sample collection cup and asked to provide a three milliliter saliva sample. Saliva samples were processed either untreated or treated with a preservative, and all samples were tested across two different COVID-19 testing platforms. After finding that virus particles remained stable in both treated and untreated saliva samples for at least 24 hours, the researchers demonstrated high concordance, or agreement, between saliva testing and NP testing. That is, results came back the same in 93% of cases.

Among the 385 samples taken, just nine had mismatched results between the NP and saliva test results. Most of the mismatches were at very low viral loads, making them clinically less worrisome, and saliva detected as many infections that NP missed, as vice versa. The researchers reported that the sensitivity of NP testing was still slightly higher that of saliva testing, with NP testing able to detect viral particles at lower concentrations. However, the researchers found that saliva testing is still likely to detect 90% of COVID-19 infections, and concluded that the minor loss in sensitivity is outweighed by the ease and safety of self-collection.

“Saliva self-collection is simply easier and more comfortable than the alternative of NP swabs, which in addition to the discomfort also require a trained medical professional,” said Ramy Arnaout, MD, DPhil, associate director of the Clinical Microbiology Laboratories at BIDMC. “In addition to it being a more convenient method, our findings show saliva collection can also be just as effective - even in a real-world scenario in which there are no eating or drinking restrictions before the saliva is collected.”

“We have demonstrated that for practical purposes, saliva is comparable to NP swabs for COVID-19 testing in outpatient settings,” added Arnaout, who is also associate professor of pathology at Harvard Medical School.

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Beth Israel Deaconess Medical Center