As scientists continue to unravel the inner workings of the novel coronavirus, more and more testing options have emerged. Several companies, like Everlywell and LabCorp, have even released COVID-19 tests that you can take on your own at home. All you need to do is swab the inside of your nose and mail the sample to a lab for processing. But to what extent do these tests compromise accuracy for convenience? How reliable are mail-in coronavirus tests?
First, a mini crash course on the two types of diagnostic tests available: PCR-based tests involve pinpointing genetic material from the virus, known as viral RNA. Specifically, they convert viral RNA to DNA and create millions copies of it. This way, they can detect even a small amount of virus, Morgan Katz, an infectious disease expert and assistant professor at Johns Hopkins Medicine, tells Mic. Then there are antigen-based tests, which detect a specific protein on the outside of the virus. “There are some being released that can be used as basically a pregnancy test. You’re just looking for this protein and get your answer right away.” They’re not yet widely available, though. Nursing homes use them, for instance, as well as the White House.
But PCR-based tests are more reliable, Katz says. They have better sensitivity, which means they have a lower rate of false negatives — results indicating that you don’t have COVID-19 when you actually do. In other words, “you’re going to miss less positives with that test.”
Most hospital and clinics obtain viral RNA for a PCR-based test through a nasopharyngeal swab, or the insertion of swab in the area between where the nasal and oral cavity meet, Katz explains. “That’s the gold standard because that’s where we expect to find the highest yield of virus.”
Mail-in tests are also PCR-based — but they tend to extract viral RNA from an anterior nasal swab, or the insertion of a swab only one to two centimeters into the nostril, Katz says. “You do decrease the likelihood of getting a good sample of the virus because you’re not going quite as far. It’s possible that the virus won’t be in that area of the nose.”
But why take a mail-in anterior nasal swab test over the gold standard nasopharyngeal swab test? If it’s hard to schedule a nasopharyngeal swab test, or the clinics that offer them in your area have long turnaround times — around five to seven days or more — you might want to take a mail-in test, which typically give you results in one to three days, Katz says. Ideally, you want a result ASAP so you can immediately start self-isolating and notifying your contacts in order to help control the virus’ spread.
A mail-in COVID-19 test might also be a good idea if you don’t feel comfortable leaving your home to take a test, or you’re unable to do so, or if you don’t have symptoms and live somewhere that doesn’t offer nasopharyngeal swab testing to asymptomatic people, Katz adds.
There are also mail-in tests that use saliva rather than anterior nasal swabs, but their reliability depends heavily on the lab processing them and the frequency of testing, which can compensate for shortfalls in sensitivity, Katz says. (She points to the NFL, which conducts saliva tests on players a few times a week.) But she recommends anterior nasal swab tests over saliva tests. Not only are the former more widely used, but “we have found higher yield of the virus in the nose rather than in the saliva.” And the results of saliva tests can vary significantly based on external variables, like whether you’ve eaten in past few hours.
If you’re considering taking a mail-in COVID-19 test, do your homework to ensure it’s legit. Run a Google search to make sure it’s received an Emergency Use Authorization (EUA) from the Food and Drug Administration, Katz says. Also check whether the lab processing the samples has received Clinical Laboratory Improvement Amendments (CLIA) certification — the test’s website should have information on the lab. Research the test’s sensitivity, which again, refers to its ability not to miss a true positive case, as well as its specificity, which refers to its ability to identify only true positive cases. “I would shoot for over 90% on both,” Katz says. You can often find this information on the test’s website or the FDA insert that comes with test.
If your results differ from what you’d expect, Katz encourages getting a nasopharyngeal swab test to confirm. Say you live with someone you know has tested positive for COVID-19, and you develop a fever, cough, and other symptoms, but your mail-in test comes back negative. Your so-called pre-test probability of infection is so high, it’s very possible you got a false negative. “That’s when I would follow up with a nasopharyngeal swab,” Katz says.
Life involves trade-offs, and COVID-19 tests are no exception. Antigen-based tests are convenient, but not that reliable, while nasopharyngeal swabs are reliable but not that convenient. Mail-in tests fall somewhere in the middle. If you want to be more confident in your results, take a nasopharyngeal swab test, if you can.