[Originally published: Oct. 20, 2021. Updated: Jan. 20, 2022.]
Note: Information in this article was accurate at the time of original publication. Because information about COVID-19 changes rapidly, we encourage you to visit the websites of the Centers for Disease Control & Prevention (CDC), World Health Organization (WHO), and your state and local government for the latest information.
As the highly contagious Omicron variant continues to sweep across the country, people—even those who are fully vaccinated—are wondering if the onset of cold and flu symptoms is really a sign of COVID-19 infection.
But COVID testing has become a complicated issue. As Omicron surged, appointments at sites where results are processed by a laboratory became difficult to find in a timely manner. So, many turned to at-home COVID-19 tests. Often called rapid tests, such kits are sold in drugstores and online, allowing people to test themselves—and get results—in their own home in a matter of minutes.
Unfortunately, at-home tests have also become difficult—if not impossible—to find. The federal government hopes to alleviate the bottleneck by requiring private insurance companies to start reimbursing customers for at-home tests and by distributing 1 billion free rapid tests to Americans.
If you can get your hands on a test, you might find the different options confusing. In which situations is a laboratory-based (often called PCR) test best? If you are traveling and need to show a negative COVID test, which should you get? Are they all equally accurate? Do some produce results faster than others? And how far does that Q-tip go up your nose with each kind?
Some of these questions are easy to answer, while others are more difficult—particularly when it comes to accuracy. That’s because all of the tests—and there are hundreds of them, from a growing number of companies and laboratories—are offered through a Food and Drug Administration (FDA) emergency use authorization (EUA). Therefore, they have not been as rigorously tested or vetted as other medical tests with full FDA approval.
And since the virus is new, all the tests are also new, meaning we have neither a long track record of comparing results, nor a true gold-standard test yet.
Furthermore, with each new variant, new questions arise. Recently, there has been discussion about whether throat swabs or saliva samples are better at detecting Omicron compared to the more mainstream method of nasal swabs—or if rapid tests are less effective at detecting Omicron.
Sheldon Campbell, MD, PhD, a Yale Medicine pathologist and microbiologist, cautions against getting caught up in what he calls mostly anecdotal data.
“There is some PCR-based data that saliva is better, but the home tests are designed to work with a nasal swab and very few responsible people would think you should replace a nasal swab with a throat swab. That’s Twitter medicine,” says Dr. Campbell, referring to a #swabyourthroat hashtag that is trending. “And I have not seen good data that antigen tests are somehow less sensitive with Omicron than they are with other strains.”
Below, Dr. Campbell and Yale Medicine infectious disease experts get into the nuances of the various available COVID-19 tests.
What are the different types of tests to diagnose COVID-19?
Unlike antibody tests, which look for prior infection, COVID diagnostic tests look for current infection with SARS-CoV-2, the virus that causes COVID-19. They are broken into two categories: molecular and antigen (more below).
A summary of their differences
Because the samples are, for the most part, collected in the same way for both, the differences between the two kinds of tests are largely in how they’re processed.
Molecular tests are generally more accurate and mostly processed in a laboratory, which takes longer; antigen tests—or “rapid tests”—are processed pretty much anywhere, including at home, in doctors’ offices, or in pharmacies. You can get antigen test results in about 15 minutes, but they tend to be less accurate.
Health care providers typically rely on molecular tests, particularly when people have COVID-19 symptoms, whereas antigen testing is often used when quick results are needed or for general screening and surveillance.
Below, we take a closer look at the two categories.
Molecular COVID tests (also called nucleic acid amplification test, or NAAT)
The first test created to detect COVID—and still the most widely used—is a molecular test called PCR (polymerase chain reaction), Dr. Campbell says. “PCR and similar tests look for the COVID virus’s RNA,” meaning genetic material that comes only from the virus, he explains. “They tend to be quite sensitive, but even among these, they are on a continuum of sensitivity and vary a whole lot.”
“Sensitivity” measures how often a test correctly delivers a positive result for people with the condition that's being tested. A test that's highly sensitive will catch almost anyone who has the disease and not generate a lot of false-negative results.
How does the test work? A molecular test looks for genetic material from the virus. The test uses sophisticated chemicals and equipment to reproduce millions to billions of copies of viral-related DNA from even the smallest sample. Because of that, the test is considered highly sensitive, leading to very few false negatives.
How is a sample obtained? Usually with a swab inserted into your nose. There are three different methods for nasal collection:
- Nasopharyngeal: A health care professional inserts a long swab deep into your nostril to collect fluid from the back of your nose.
- Mid-turbinate: This method, which someone can be coached to do themselves or is done by a professional, involves placing a soft swab straight back into the nostril (less than one inch) to collect a sample.
- Anterior nasal swab: This test, which can either be self-administered and possibly supervised by a trained health care provider, or done by a health care professional, involves putting a swab three-quarters of an inch into the nostril and twirling it around at least four times to get a sample.
In general, the deeper you go for a specimen, the greater the sensitivity, says Richard Martinello, MD, a Yale Medicine infectious disease expert. “But, we’ve found it’s much more comfortable to do mid-turbinate or anterior nasal swabs, and they provide a reasonable degree of sensitivity,” he adds. “It’s a compromise of sorts, but it does allow us to simplify the collection process.”
Other collection methods include:
- Oropharangeal (throat) swab: A trained health care provider collects a sample using a swab to the back of the throat.
- Saliva: You drool into a sterile, leak-proof cap container. For now, this type of testing is only offered at select locations.
How is the test processed? Most specimens are sent to laboratories.
Where can you get one? Molecular tests are offered at pharmacies, doctors' offices, and designated testing locations, such as health clinics, as well as locations set up by private or state and local public health systems.
How quickly can you get results? Because the tests are sent to a lab, it depends on lab capacity. Results are typically available within a range of one to seven days, depending on your location.
“Currently, Yale New Haven Health System is turning around about 76% of community test results within 24 hours and just over 95% within 48 hours,” says Scott Roberts, MD, a Yale Medicine infectious disease specialist.
How accurate are they? According to the Centers for Disease Control and Prevention (CDC), laboratory-based tests, such as PCR, have a “generally high” test sensitivity.
“PCR tests are considered the most accurate available,” Dr. Martinello says. “But because these tests are highly sensitive and specific, there is still a risk for a false positive.”
But limiting false negatives might be extremely important, especially with the rise of more transmissible variants. "It’s actually true for those who have—and who don’t have—symptoms, but if you do have symptoms, a PCR test is more likely than an antigen test to pick up an infection accurately,” says Dr. Campbell.
Antigen COVID tests
Whereas molecular tests require specialized equipment for processing samples, an antigen test is simpler, because it requires smaller devices that are easy to transport. Their design is similar to, say, a pregnancy test.
How does the test work? Antigen tests search for pieces of protein from the SARS-CoV-2 virus. The sample you provide is treated with a reagent and analyzed on the spot by a health care professional. Unlike molecular tests, these require a higher level of virus in the test sample before the test will turn positive. This means that an antigen test may sometimes lead to a false negative.
How is a sample obtained? As with molecular tests, a sterile swab is inserted into your nose or throat to obtain a specimen (see details above)—although throat swabs may be less common these days.
How is the test processed? The sample is self-applied to a test strip or cartridge. Similar to a home pregnancy test, results show a colored line to signify positive or negative.
Where can you get one? Antigen tests are offered in pharmacies, doctors' offices, and can be purchased to use at home.
How quickly can you get results? Results are typically available in 10 to 15 minutes.
How accurate are they? According to the CDC, antigen test sensitivity varies depending on the time in the course of one’s infection, but is considered to have “moderate to high” sensitivity during peak viral load. Compared to molecular tests, antigen tests are more likely to generate false negative results, especially when performed on people who don’t have symptoms.
ID NOW COVID Test
Complicating things a bit is the availability of ID NOW, a rapid molecular test used by some testing locations, such as pharmacies, that can read results on-site—in about 15 minutes.
According to the CDC, point-of-care tests (such as those administered at a drug store, including ID NOW), have a “moderate to high” test sensitivity.
But where does it fit in the molecular vs antigen test result accuracy spectrum?
“ID NOW is not a completely different thing than PCR, it’s just on the lower sensitivity end of the spectrum,” Dr. Campbell says. “So, more accurate than an antigen test.”
What about home tests?
The home tests that give instant results are all antigen. However, there are home kits that require mailing a sample to a lab that use molecular technology. Whereas testing at pharmacies and doctors’ offices is typically free or covered by insurance, your insurance company may not cover the cost of a home test, which may cost anywhere from $24 for a set of two to $38 for one.
Home tests are useful if you need an immediate answer, Dr. Campbell says. “But the tricky part is that the cost can pile up and people don’t always do the test correctly, either,” he says. “It’s extremely important to follow the directions carefully.”
And since they are less sensitive than a molecular test, the FDA recommends doing serial testing—or taking multiple tests—over several days to improve the chance of catching asymptomatic infections.
But given the current shortage of tests, Dr. Campbell advises using them sparingly.
“If you can access a home antigen test, I would wait to take one until about three days after a known COVID exposure. If it’s negative, I would do it again at days five and seven,” he says. “Even that is a lot of tests. So, depending on the degree of your exposure and if you turn symptomatic, you might not need to keep testing, especially if you are vaccinated and a ‘low-risk’ person. Rather, test again when your symptoms have gone away and you are ready to go back to work.”
The overarching principle of testing in the medical world, Dr. Campbell explains, is that you should only test when a result will change what you do. Given the high case rate levels right now, if you are exposed to COVID and then develop symptoms, chances are you have the virus, he says. If your symptoms can be managed safely at home and you can isolate, then that is what you should do.
Another thing to remember, Dr. Campbell says, is that if an antigen test comes back positive right now, it is most likely accurate.
“People who have a positive antigen test right now are almost certainly infectious and contagious. People who are antigen negative at the very worst are less contagious. We can’t say they are absolutely not contagious because the studies are hard to do, but they are less contagious for sure,” he says. “When the incidence of disease is really low, false positives start to outnumber the true positives. But at this moment, when incidence is so high, false positives are extremely unlikely.”
What type of test should I get?
If you are exhibiting a fever, cough, trouble breathing, or other COVID-19 symptoms, you should get tested, regardless of vaccination status, health experts advise. If testing is available, consider testing three to five days after a high-risk exposure, adds Dr. Campbell.
Deciding which type of test to get can be challenging.
“A lot of this depends on access and what is readily available to you. We are thankful to have rapid antigen tests, but if you don’t have symptoms, their sensitivity is limited and we know that 40% of people who are contagious with COVID are asymptomatic,” Dr. Martinello says. “A NAAT test is more sensitive, but a lot still depends on the quality of the specimen.”
Still, for the diagnosis of people seriously ill (with a presumed case of COVID-19), doctors will generally use a PCR test, because false-negative tests might result in inadequate treatment.
If you are traveling, you might be required to get tested, too. The location you are visiting might require a certain type of test and approved testing locations as well.
“I think the test you can get makes the most sense for travel right now. If you can get PCR, great. If not, get an antigen,” Dr. Campbell says. “But you do want to test because you don’t want to be the one person who infects everyone else on the airplane.”
School and workplaces
Schools may have their own rules for how they conduct COVID-19 testing. But for surveillance, such as at schools or workplaces, antigen tests work well, Dr. Campbell says.
“Say you are testing kids in a school twice a week in perpetuity. You can do it much more quickly and easily and for less cost if you use antigen tests,” he says. “You want to answer the question of if kids are infectious now or not. Would PCR be better in that setting? Yes, but you won’t get the answer back in a day necessarily, and you would spend a lot of money to find a few positives. Sometimes, speed is the most important thing.”
General peace of mind
Some people might like to do regular COVID tests for peace of mind. Say you are vaccinated but plan to attend a crowded event and then visit an immunocompromised or elderly relative.
“If you are going to do something risky and then visit your 90-year-old mom, then what you really want to do is test before the event and make sure you’re not the person spreading virus,” he says.
After that, the timing of what to do next is complicated.
“If you went to a crowded concert and are worried about COVID, you don’t want to take any COVID test—molecular or antigen—the next day. You should wait three to five days after potential exposure,” says Dr. Campbell. “We think that you have to have a fair amount of virus present to be infectious to others, and we know that in the course of infection, the viral load goes up and down.”
If you have a limited number of tests, you should use them right before you visit vulnerable friends or relatives—or right before you go to an event with lots of people, adds Dr. Campbell. “Use them to keep from spreading infection,” he says.
Will any of the tests tell me which variant I have?
None of the COVID-19 tests you take, either at home or at a pharmacy, will tell you if you have a variant, such as Delta or Omicron. Detecting variants requires genetic testing done in a lab. Across the country, a selection of positive COVID-19 samples are sent to specialized labs, where they are anonymously sequenced to identify variants so that public health officials can monitor COVID-19 trends.
And not all positive specimens are tested; only sample amounts are taken. So, if you hear that 75% of cases in an area are a particular variant, for example, that reflects a computation based on the number of samples tested—and it is just an estimate. By January, Omicron made up about 95% of COVID-19 cases in the U.S.
Are there tests that look for COVID-19 and the flu?
Since the symptoms of COVID-19 and the flu are similar, it’s helpful to know there are molecular tests you can have that detect each virus using a single sample. In fact, there are even tests that diagnose COVID-19, flu, and RSV (respiratory syncytial virus), a virus that causes common cold symptoms, at once.
Such tests are offered at doctors' offices and clinics and need to be sent to a lab, and should come back in a day or less. These tests will become available closer to flu season, in October.
Even for physicians, COVID-19 testing can be confusing.
In the end, it’s best to remember the basics: get your vaccine, when in doubt wear a mask and social distance, and stay home and away from others if you feel sick, Dr. Campbell says.
The federal government launched a website for people to order free COVID-19 test kits (four per household) to be shipped to their homes. Visit the site by clicking here.
Note: Information provided in Yale Medicine articles is for general informational purposes only. No content in the articles should ever be used as a substitute for medical advice from your doctor or other qualified clinician. Always seek the individual advice of your health care provider with any questions you have regarding a medical condition.