In case you missed it: You can now get tested for the coronavirus in the comfort of your own home.
This is great news, especially for people who don’t have access to a testing site. Currently, these portable tests come in two flavors. The first is test-by-mail kits, which allow patients to swab their noses at home and mail them to a laboratory for a result in a day or two. The other types are called at-home tests, which give an answer on the spot.
Currently, the Food and Drug Administration has authorized dozens of test-by-mail kits, and three at-home tests.
These tests are not nearly as accurate as those taken in a clinic, but experts say coronavirus tests that can be done at home play an important role as the country continues to reopen. “They get actionable information in people’s hands quickly,” said Jennifer Bacci, an assistant professor at the University of Washington School of Pharmacy.
Of course, no coronavirus diagnostic test is 100 percent accurate. Even the gold-standard nasopharyngeal swab, given at many clinics, can return a negative result even though you might be carrying the coronavirus. And these tests only inform you about a single point in time. But even if home tests may be less accurate, they can quickly alert people if they test positive.
Certainly the market for home test kits will likely grow, said Gigi Gronvall, a public health expert at Johns Hopkins University. But with more options, consumers will need to learn what test is best for them.
Here are some key questions to consider when deciding on an at-home testing kit.
What are the trade-offs between mail-in kits and fully at-home tests?
Test-by-mail kits require users to purchase a kit, take a sample at home and ship the swab back to a lab. These kits take more processing time and use a method called polymerase chain reaction, or P.C.R., to detect the coronavirus.
P.C.R. works by identifying and magnifying specific gene sequences. “It can take a very small signal and amplify it,” to detect smaller amounts of the virus, said Dr. Gronvall. These tests are highly sensitive, picking up positive cases nearly all the time (accuracy varies by lab, and false negatives can be as high as 20 percent). “A negative P.C.R. isn’t perfect, but it gives a high degree of assurance,” said Dr. Ashish Jha, dean of the Brown University School of Public Health.
Fully at-home tests, such as those made by Ellume and Abbott, require users to swab their noses and drop the swabs in a liquid. The tests provide an answer in as little as 15 minutes for the Abbott test and 20 minutes for Ellume.
These tests look for antigens — parts of microbes that cause an immune response. Unlike P.C.R., antigen tests do not amplify signals, which makes them faster but less accurate. These rapid antigen tests, Dr. Gronvall said, are good for measuring how contagious you are. “If you test positive on that, you really need to isolate,” she said, and get a clinical swab done to confirm the results.
False negatives, however, are much more common with antigen tests, meaning infected people might think they are virus free, especially if they are not having symptoms.
“The sensitivity of these tests tend to be pretty bad,” said Dr. Yvonne Maldonado, an infectious disease specialist at Stanford University School of Medicine. If users have symptoms, the BinaxNOW antigen test has a 64 percent chance of correctly spotting the virus (and about half that in those without symptoms). Accuracy for some antigen tests in asymptomatic individuals can be less than 50 percent — worse than flipping a coin, she said.
Remember, any test’s ability to detect coronavirus depends on how much virus is in the location of your body where you are taking a sample. Tests taken early, say, hours after a potential virus exposure, have a higher chance of resulting in a false negative.
What home test should you use?
If you’re asymptomatic, you may have a smaller amount of virus in your body. In this case, experts said that your best bet for an accurate test is to use a test-by-mail kit because P.C.R. will be able to amplify lower levels of virus.
If you have symptoms, either a P.C.R.-based test or an antigen test will likely be able to confirm you have it. When choosing an antigen test, Dr. Jha said to look for whichever option at your disposal has the highest sensitivity, which refers to a test’s ability to detect the virus. Look for a sensitivity rating from 95 to 99 percent, he said.
Turnaround time is also important. Antigen tests are less accurate but offer an answer much faster without having to mail a sample. Results of either test should always be confirmed by a clinical test, said Dr. Maldonado.
Costs, too, may play a factor. Test-by-mail kits can cost $100 or more and may not be reimbursed by insurance companies. “Many patients have encountered unanticipated bills or red tape when seeking reimbursement for mail-in coronavirus testing, even though insurance companies are obligated to do so,” said Dr. Marisa Cruz, head of clinical affairs at Everlywell, a company that makes at-home health tests, including one for the coronavirus.
Antigen tests, on the other hand, are a fraction of the cost, currently ranging from $25 to $50.
What should you check for on the box?
Make sure the home test or collection kit you’re looking to buy has an emergency use authorization from the F.D.A. (it will be printed on the box) and that the company works with certified lab partners. Also look for tests that offer a telemedicine consult, advised Dr. Cruz, so you can discuss your diagnosis and next steps.
How should I interpret a result from an at-home coronavirus test?
Following the test kit instructions is key to getting a reliable result. “A specimen that is not collected correctly may lead to false negative test results,” said Dr. Cruz. Imperfect swabbing technique, or swabbing only one nostril, may increase the risk of less accurate results. And samples for test-by-mail kits should be shipped the same day they are collected; the less time in transit, the better. Samples sent on weekends or holidays may be delayed, though some use FedEx and overnight shipping.
If you test positive on either a mail-in P.C.R. or an at-home antigen test, you are likely to be infected and presumed contagious, said Dr. Bacci, so isolate from others and continue to monitor your symptoms. Repeat testing can help track the disease course, if, say, someone goes from being asymptomatic to displaying symptoms.
Negative results are more likely to be wrong than positive ones. “A negative result does not necessarily mean you do not have Covid-19, which is the same interpretation for either an at-home test, a mail-in test or one offered in a doctor’s office,” said Dr. Cruz. Continue to wear masks, socially distance and practice good hygiene, especially if you have symptoms or known contacts with others with Covid-19.
When would a test be inappropriate to use?
Dr. Gronvall is concerned that some people are using at-home tests after they’ve been vaccinated to make sure that the vaccine has worked. But neither the P.C.R. nor antigen-based tests will be able to discern whether the vaccines have built up immunity in your body.
That’s because the vaccines encode for snippets of the virus and not the entire sequence. The P.C.R. and antigen tests search for a different portion of the virus from what’s included in the vaccines.
“These tests are not going to tell people if the vaccine is effective,” she said.
What does the future of at-home testing look like?
Beyond saliva and nasal swabs, some scientists are looking to develop devices that look like breathalyzers to detect chemicals in an individual’s breath that correspond to coronavirus infection. “We’re looking for the body’s response to infection and disease,” said Pelagia-Iren Gouma, a materials engineer at The Ohio State University.
Dr. Gouma and her colleagues are testing a small breathalyzer they have developed that can be used for up to one year and would cost perhaps a few dollars per device. Users would get an answer in 15 seconds, and the test appears to be accurate 96 percent of the time and can be reused, Dr. Gouma said. The device was submitted to the F.D.A. and has been awaiting emergency use authorization since September.
Experts hope that as the market for at-home testing expands, the options will grow and become cheaper. The cheaper the tests are, the more likely the government will subsidize them and consumers will buy them for routine testing. And the more testing, the better. As the world slowly reopens, home-based tests will help people navigate their decisions.
Wudan Yan is a journalist based in Seattle, Wash., writing about science and society.