Understanding the Different Coronavirus Tests
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With cases of COVID-19 rising across the country, it’s more important than ever that people with symptoms—such as fever, cough, shortness of breath, loss of smell, and diarrhea—get a diagnostic test for the virus, says Yuka Manabe, MD, a professor of medicine and an expert in infectious disease at the Johns Hopkins University School of Medicine in Baltimore.
Diagnostic tests identify who is actively infected with SARS-CoV-2, the coronavirus that causes COVID-19. They are different from antibody tests, which may tell you if you were infected at some point in the past.
Even if you feel fine, the Centers for Disease Control and Prevention advises getting tested if you are at higher risk for contracting the virus—for example, if you were in close contact with someone who’s infected. Your healthcare provider or public health officials may also recommend getting tested if there is a high rate of COVID-19 in your area and you attended a gathering where not everyone was wearing masks and staying at least 6 feet apart.
“The superpower of SARS-CoV-2 is that it can spread when the person is asymptomatic,” Manabe says. Identifying those people and having them isolate for at least 10 days—as recommended by the CDC—is crucial to help stop the spread of the disease.
The optimal test depends on your individual situation, says David Pride, MD, associate professor of pathology at the University of California, San Diego, but you might not always have a choice. “If you are concerned, go get tested,” he says. “While there are some tests that are better than others, if only one is available to you, then use it.”
Here’s what you need to know about diagnostic tests for COVID-19.
Types of Tests
Tests for identifying active coronavirus infections fall into two main categories—tests that provide rapid results on site, and those that have to be processed in a lab. Each type of test excels in different circumstances, and versions of each have been authorized for home use.
Lab-based tests: These are “molecular tests,” many of which are known as PCR tests, that look for tiny bits of genetic material from the virus and duplicate it many times over until it reaches a detectable level. That process takes several hours in a laboratory with specialized equipment.
Lab-based molecular tests are the most accurate way to tell if you have been infected by the coronavirus, says Kelly Wroblewski, director of infectious disease programs at the Association of Public Health Laboratories.
The only downside is the processing time. Currently, you can expect to hear results in a few days, although wait times may be longer or shorter in some areas. If you have the option to choose from multiple testing sites, Wroblewski advises checking to see which has the shortest estimated turnaround time. You can also “ask specifically, ‘If I’m positive, will I hear back in a particular period of time?’” Wroblewski suggests.
Rapid, on-site tests: The most widely available of these are antigen tests, which detect specific proteins, known as antigens, on the surface of the virus, though the FDA has also authorized several rapid molecular tests.
With a rapid test, you should get results on the spot or on the same day. But you sacrifice some accuracy for speed. In general, antigen tests are less sensitive, Wroblewski says. So you need a higher amount of virus in your specimen to detect it. They can yield falsely negative results early in the course of an infection, when the level of the virus in the body is still low.
“They [rapid tests] perform pretty well in people who have symptoms but can be less reliable in those who don’t,” she says. In fact, if you have COVID-19 symptoms and receive negative results from a rapid test, your healthcare provider may recommend following up with a lab-based test.
Antigen tests do a good job of identifying people harboring high enough levels of the virus to infect others—at least at the moment they took the test—says Pride at the University of California, San Diego. So regularly testing people in group settings, such as at schools or factories, can catch those who are infectious and reduce the risk of an outbreak.
Home tests: These include both molecular and antigen tests. The FDA has authorized three tests that can be performed entirely at home, as well as more than two dozen home-collection kits, which let you swab the inside of your nose or spit in a tube at home, then send your sample to a lab by expedited shipping.
The three complete home tests are the Lucira COVID-19 All-In-One Test Kit, a rapid molecular test that costs around $50 and requires a doctor’s prescription; the Ellume COVID-19 Home Test, an antigen test that will cost about $30 and does not require a prescription; and the BinaxNow COVID-19 Ag Card Home Test, an antigen test that will cost about $25 and requires a doctor’s prescription. None of these completely at-home tests are yet available nationwide.
The home collection kits are widely available, and many can be ordered online.
How Samples Are Collected
There are three main types of collection methods.
Nasopharyngeal swab: A healthcare worker inserts a 6-inch swab through your nostril to scrape cells from the nasopharynx, the upper part of the throat behind your nose. “It’s the most sensitive sampling method because it gets right to where the virus tends to replicate,” Wroblewski says. But it’s a little uncomfortable and requires trained personnel to administer.
Nasal swab: This method, which swabs an inch or so inside the nose, is more comfortable than the nasopharyngeal swab and is simple enough to do yourself. Guidelines from the Infectious Diseases Society of America concluded that shallower swabs work almost as well as the “brain tickling” versions.
Saliva: For this method, you’ll be asked to spit into a collection tube. While research has shown that saliva testing is promising and easy, studies of its reliability have been somewhat mixed. “In our lab, we’ve never been able to demonstrate that it’s as sensitive as a nasal swab,” Pride says. If you have a choice, he recommends a test that collects samples from inside your nose.
Getting Tested Safely
To minimize the risk of infecting someone else with the virus (or vice versa), Manabe advises finding a drive-thru testing site or one where you can make an appointment. “If you must stand in line, it’s better to wait outside than inside,” she says.
Waiting inside should still be fairly safe as long as everyone is wearing masks, and staying 6 feet or more apart. And a shorter wait inside is safer than a long wait.
You should also avoid touching your face, and wash or sanitize your hands as soon as you leave the testing area, Pride says. “If someone near you isn’t following those protocols, don’t be afraid to speak up or move so that you are not exposed.”
If you want to avoid venturing out to a clinic or test site, consider using a home collection kit. Just make sure you follow the instructions exactly.