August 24, 2020
Do COVID-19 saliva tests work?
BY Cynthia DeMarco
With the Food and Drug Administration recently issuing emergency use authorization for a new method of saliva sample processing, you may be wondering about COVID-19 saliva tests.
Do these saliva-based COVID-19 tests work? How accurate are they? And how do they compare to COVID-19 nasal swab tests?
We asked Keyur P. Patel, M.D., Ph.D., a molecular diagnosis expert and the medical director of our Molecular Diagnostics Laboratory.
How do COVID-19 saliva tests work?
Coronavirus particles are shown to be present in saliva as well as in respiratory droplets, so they’re detectable in saliva using the same kind of analysis we perform on fluids collected with nasopharyngeal swabs (also called COVID-19 nasal swab testing). Laboratory testing aims to identify the presence of viral genome in the saliva sample using molecular methods that are similar to those used for nasal swab testing.
How much saliva is needed for a COVID-19 saliva test? How is the saliva collected?
Only a very small amount of saliva is needed to conduct a COVID-19 saliva test — less than a quarter of a teaspoon. So, patients are usually asked to let it pool in the bottom of their mouths for a few seconds without swallowing, then lean forward and let it drip into the small, sterile container they’re provided.
Spitting is something testers try to avoid, because that can generate aerosols. And they don’t want anyone clearing their throat or coughing first, either, because testers are looking for saliva, not mucus or phlegm from the back of the throat.
People being tested are also asked avoid drinking water for at least 10 minutes and not to eat or drink anything else for about 30 minutes beforehand. That way, the samples are as clean and contaminant-free as possible.
Are COVID-19 saliva tests safe?
Yes, it is completely safe for the person providing the saliva sample. And it’s not invasive at all.
It’s also fairly safe from the point of view of the person collecting it, assuming they adhere to all the usual safety protocols for clinical staff. Because unlike nasal swab tests, which can make people sneeze or cough, this is just having someone drool into a test tube.
Collecting a nasal swab from someone with a suspected COVID-19 infection is a pretty involved procedure. We have to wear gowns, gloves, masks and other equipment to protect ourselves and the patients we’re testing. But with saliva testing, the person collecting the sample doesn’t have to be actively involved, or even near the person providing it. It’s a case of, “You collect, and I’ll go stand over there, watching and providing instructions.”
Are saliva-based COVID-19 tests accurate?
An appropriately designed saliva test can be highly accurate. The FDA has recently issued emergency use authorization for saliva-based tests for clinical use based on a high level of accuracy that’s comparable to COVID-19 nasal swab testing.
When it comes to diagnosing COVID-19, which is more effective: nasal swab testing or saliva testing?
Our approach to COVID-19 has been evolving as more clinical data is being generated. Since the beginning of the pandemic, COVID-19 nasal swab testing has been a popular choice as a sample type among the list of upper and lower respiratory specimens to be tested in CDC’s guidance.
But recent data and FDA approvals have shown that saliva can do an equally good job in an appropriate clinical setting. That means in the general population, only up to 5% of patients who receive a negative result from a saliva test might need to have a nasal swab test done, too, and then, it’s only if there’s a strong clinical indication that they have the coronavirus. There is no substitute for a sound clinical judgement.
At present, we continue to use nasal swab testing for COVID-19 testing at MD Anderson, but we are actively working on developing laboratory testing for saliva.
What are the chances of getting a false negative or a false positive from a COVID-19 saliva test?
Currently, COVID-19 saliva-based testing is being compared to nasal swabs. In the data submitted to FDA for emergency use authorization, neither the study conducted by Rutgers University nor the study conducted by Yale University found any false positives, as compared to COVID-19 nasal swab testing.
The Yale study only missed about 5% of the positive cases, or 2 out of 37. The Rutgers cohort had no false negatives. So, that’s very promising. In addition, a recent peer-reviewed publication from the University of Utah involving 354 patients showed similarly high positive percentage agreement (94%) and negative percentage agreement (98%) between COVID-19 saliva and nasal swab testing.
We need additional studies to evaluate the role of saliva testing in individuals who are suspected to have COVID-19 based on a history of travel or exposure, but who aren’t showing any symptoms.
How long does it typically take to get results from COVID-19 saliva tests?
It varies by the laboratory and specific clinical setting. The amount of time that it takes to transport the specimen from the point of collection to the testing laboratory can be a major component of the overall turnaround time. Once a sample is received, the time it takes the lab to analyze it tends to be relatively constant and depends largely on the number of samples received on a daily basis relative to the overall testing capacity.
Having said that, saliva samples do offer the possibility of using laboratory methods that reduce the turnaround time by a few hours, compared to COVID-19 nasal swab testing. In practice, the laboratory may choose to keep the same testing protocol for both COVID-19 nasal swab testing and saliva testing for a variety of reasons, and the overall turnaround time may be similar.
When should someone get a COVID-19 saliva test versus a nasal swab test?
Recent FDA emergency use authorization shows COVID-19 saliva testing offers comparable results when compared to COVID-19 nasal swab testing. Right now, it’s not a question of one over the other. It’s more a question of availability, as there are a limited number of clinical laboratories that offer the saliva testing under FDA emergency use authorization. At MD Anderson, we’re testing patients with COVID-19 nasal swab testing.
Obviously, saliva testing involves a much less uncomfortable method of collecting a fluid sample than nasal swab testing. And ultimately, saliva testing could be a game-changer, in that it allows more people to be tested, while simultaneously keeping health care professionals safer.
But the indications for needing a COVID-19 test are the same, regardless of what type of test is being administered.
Where are COVID-19 saliva tests available?
Right now, COVID-19 saliva tests are available at a limited number of laboratories. However, this is likely to change, as more clinical laboratories and commercial assay manufacturers are actively looking to develop clinical saliva testing solutions.
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Request an appointment at MD Anderson online or by calling 1-877-632-6789.
Saliva testing could be a game-changer.
Keyur P. Patel, M.D., Ph.D.
Physician & Researcher