Experts Explain COVID-19 Testing

Vaccines are on the way, but in the meantime, those getting tested after a COVID-19 exposure should know that tests are not equally effective in detecting the virus. That’s according to experts, including Mayo Clinic's Dr. Bobbi Pritt, who told BJT that molecular-based tests are the most successful at detecting the presence of SARS-CoV-2, the virus that causes COVID-19. She explained that detection requires an incubation period of three to four days following exposure. It can take two to 14 days for symptoms to appear, Pritt said, although most people become symptomatic five or six days after exposure.

“There’s no evidence to suggest your viral loads are high enough one day after your initial exposure to be picked up by even the most sensitive tests,” added MedAire product director for airport and testing services Alexander Smith. “And there are a few variants of molecular tests. PCR isn’t the only one, it’s just the most popular. They’re incredibly sensitive and will pick up even the slightest hint of a viral particle in that genome. But...viruses are tiny and they have to replicate enough and they have to get to that area where they’re actually collecting that sample.”

Antigen testing is a less effective means of COVID-19 testing, explained Pritt, who is director of Mayo Clinic’s clinical parasitology laboratory. She explained that the antigen test is “good” at detecting coronavirus while a molecular test is “very good.” An antigen-based test is “going to miss a certain number of people who are infected,” Pritt said. 

Smith explained that the throat is the most likely place where the virus will replicate in the body. That’s why clinicians largely use an oropharyngeal or nasopharyngeal swab, which can be uncomfortable for the patient. The swab test is the most widely used by far. “There’s going to be less virus on the tip of your tongue than in the back of your throat,” Smith added.

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However, a few molecular tests that have been approved by the Food and Drug Administration can detect the virus in saliva. Smith said these non-invasive tests are “very accurate.” MedAire is offering a COVID-19 testing program for aircrew, passengers, and personnel using an FDA-authorized PCR saliva test.

There’s really no difference between the effectiveness of so-called quick tests and those that take days to produce results, as long as the tests are molecular-based. The speed with which the results are delivered is mostly a matter of whether a clinician has on-site, dedicated access to laboratory equipment or has to use a third-party provider to test the sample. Smith noted that the testing equipment is expensive and the tests themselves “aren’t cheap.” It’s why, for example, a large hospital might be able to offer a rapid PCR test while a doctor’s office may not.

Mayo’s Pritt noted that just because someone tests negative for COVID-19 doesn’t mean they are free and clear to do whatever they want. “Testing in and of itself is not a fail-safe,” she said. "It doesn’t give you a get-out-of-jail-free card that lets you go and expose [yourself] to reckless behaviors, go out to a bar, to a restaurant, and not socially distance. Because no test is perfect and because there’s that incubation time where someone could be infected but not know it and a test would not be able to detect it, it’s clear that testing has to be done in addition to mask-wearing and social distancing.”