Experts Explain Covid-19 Testing and Test Effectivity

 - December 7, 2020, 1:09 PM
Photo: United Nations

This story is part of AIN's continuing coverage of the impact of the coronavirus on aviation.

Flight crews should expect several days to pass from a Covid-19 exposure before they can be accurately tested for infection, and they should know that not all tests are created equal. That’s according to experts, including Mayo Clinic's Dr. Bobbi Pritt, who spoke with AIN and said molecular-based tests are the most effective at detecting the presence of SARS-CoV-2, the virus that causes Covid-19. She explained that detection requires an incubation period of anywhere from three to four days following exposure. It can take anywhere from two to 14 days for symptoms to appear, Pritt said, although for most people they become symptomatic five or six days after exposure.

“There’s no evidence to suggest your viral loads are high enough after one day or after your initial exposure to be picked up by even the most sensitive of tests,” added MedAire product director for airport and testing services Alexander Smith. “And these molecular tests like [polymerase chain reaction or PCR] tests—there are a few different variants of molecular tests, PCR isn’t the only one, it’s just the most popular—they’re incredibly sensitive and they will pick up even the slightest hint of a viral particle in that genome. But there has to be enough; viruses are very 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. “It’s always best if you could use the most sensitive method that’s going to have the highest likelihood of detecting someone who’s infected.”

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

However, there are a few molecular tests approved by the Food & Drug Administration (FDA) that can detect the virus in saliva, which Smith said are obviously not invasive nor painful and are “very accurate.” MedAire is offering a Covid-19 testing program for aircrew, passengers, and personnel using an FDA-authorized PCR saliva test.

In terms of so-called quick tests, there’s really no difference between their effectiveness and those that take days instead of minutes for the 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 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 you can 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.”