Home » Saliva-Based PCR Test Just as Effective as Nasal Swabs for COVID Detection

Saliva-Based PCR Test Just as Effective as Nasal Swabs for COVID Detection

by Coffee Table Science

Processing covidSHIELD tests in one of SHIELD Illinois’s labs. Credit: Fred Zwicky/University of Illinois

Megan, an Illinois lady, took her children to be tested after they were exposed to COVID. A quick test from the local drugstore could have sufficed, but her son Brennan despised the nose swab—it tickled and it was simply not a pleasant experience. So, Megan took her kids to a testing facility, where her son could simply spit into a tube to submit his sample. Like other COVID tests, The findings were available In less than 24 hours.  

COVID testing has come a long way from the early days of the pandemic when patients had to go to a doctor’s office and have deep “brain-tickling” nose swabs and wait days for results. The University of Illinois at Urbana-Champaign (UIUC), Yale University and Rockefeller University, have developed saliva-based tests on their own. These tests are less intrusive, faster to run, and, in some situations, more sensitive than nasal tests. 

While nose swabs might be difficult to use on a regular basis, “it’s fairly easy to provide a saliva sample repeatedly,” says Rebecca Lee Smith, an infectious disease epidemiologist at UIUC and a scientific advisor for the group that developed covidSHIELD, the saliva test Brennan undertook. 

CovidSHIELD is a highly sensitive polymerase chain reaction (PCR) test that requires amplifying genetic material from SARS-CoV-2, the virus that causes COVID, to a detectable level. A laboratory is generally, but not always, used for PCR analysis.

When a covidSHIELD saliva sample arrives at the lab, scientists place it in a hot-water bath to kill any viruses that may be present and neutralize saliva components that may interfere with the test. Now the sample is ready for amplification. Short bits of genetic material called “primers” are introduced to the sample. Primers are “a small ‘idea’ of what the virus looks like in the RNA [genetic material],” according to Smith. The virus’s RNA is transformed into complementary DNA (cDNA). The primer seeks to produce a copy of the virus’s cDNA by connecting to it. Any cDNA is amplified through these copies when the procedure is repeated, or “cycled,” until it can be easily detected and designated a positive sample.

A typical RNA extraction step can be skipped with the covidSHIELD technique. Instead, the viral RNA is released via heat and chemical treatments. As a consequence, the test’s creators can guarantee findings in less than 24 hours. According to Beth Heller, a spokesman for SHIELD Illinois, a nonprofit entity of the University of Illinois System that administers the exam throughout the state, the typical turnaround time for results is even faster. “Right now, from sample collection to results, we’re averaging 13 hours,” she adds.

The time it takes to acquire a COVID test result is an important issue to consider when considering any diagnosis. It’s part of a larger discussion concerning the use of PCR vs fast antigen tests, as well as overall testing sensitivity. Antigen testing is typically performed using a nose swab, but researchers are working on a more reliable saliva-based version. They function by utilizing a specific testing strip to look for viral antigens, which are pieces of the virus that cause an immune response.

Antigen tests’ proponents believe that because of their quickness and low cost, they are more suited as a public health tool. “A PCR test, especially one that requires sending to a laboratory, should really be reserved for medicine,” says epidemiologist Michael Mina, who has advised President Joe Biden and his administration on testing and is the chief science officer of eMed, a company that sells antigen tests that are verified through a telehealth platform.

PCR tests’ extreme sensitivity is one of their main problems in a public health context. “I don’t want [someone] to have a positive test if [they are] not infectious anymore because if they’re positive, I’m going to tell them to isolate for 10 days,” Mina says. “And if they’re not infectious anymore, me telling them to isolate for 10 days is bad public health.”

Saliva-based testing are useful for determining answers to inquiries like “I believe I was exposed this weekend.” Is it possible that I’m infected? “Will I be contagious the next day?” Smith explains. Antigen testing, on the other hand, is beneficial for detecting whether or not someone is infectious. “Rapid antigen tests are great for quickly determining whether or not you have these symptoms. “Am I contagious right now?” she wonders.

Because of their sensitivity, saliva PCRs may still show up as positive after someone is no longer contagious. According to Smith, an antigen test may be more suited at this stage for detecting whether someone is no longer infectious. “I don’t advocate any PCR [test] for leaving isolation,” she continues, “but antigen testing can be quite helpful.” 

According to Smith, the same saliva-based PCR method might be used to detect additional diseases than COVID in the future, such as influenza or respiratory syncytial virus. “We’re starting to work on a project for what’s called a multiplex PCR,” she explains. “We would investigate other respiratory infections that produce comparable symptoms rather than merely testing for COVID-19.”

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