"They all have their merit in different ways," says Dr. Labus. "The PCR is the most accurate test and that's the one that we've focused on. That's how we're testing people nationwide because that gives us the best answer. The antigen tests can be a big benefit because they're quick, but you trade off a lot of that accuracy for speed, which is a problem. If we're still missing a lot of cases to get an answer quickly, that's a problem for public health. The antibody tests are interesting in a research setting if we're trying to study what percent of the population has been infected. But beyond that, it really doesn't tell us much."
- Brian Labus, PhD, MPH, infectious disease epidemiologist and public health professor at the University of Nevada, Las Vegas
As of September 10, there are 247 COVD-19 tests authorized by FDA under emergency use authorizations. These include 197 molecular tests (including PCR tests), 46 antibody tests, and 4 antigen tests.
PCR tests
Dr. Labus says PCR tests, or polymerase chain reaction tests, look for the genetic material of the virus. The specimens collected for the test come from the nose and mouth. If you test positive, it means the virus is currently active in your body.
The first COVD-19 PCR test we were introduced to was the nasal pharyngeal swab, which involved what feels a bit like a Q-tip rubbing against your brain. "It's really uncomfortable for the person that's having the test done," he says. These tests were initially so invasive because there wasn't enough data showing whether accurate results could be achieved from a swab lower in the nasal cavity. Now, there are much more tolerable options. "We now have tests where you can just use [a Q-tip] in your nose, so essentially swabbing your own nose with a Q-Tip—it's no more invasive than that," he says. There is also a saliva test where all you do is spit in a tube.
"And all three of those [tests] are equivalent in terms of the results they get and how accurate they are," he says.
Though this is the most accurate way to test for COVID-19, it's not 100 percent accurate.
"Even with the PCR, we know 20 percent or 30 percent of the time, you can get a false negative," he says. "That's especially true if you test too early. So if I'm exposed and I get tested the next day, it's too early to detect anything. Even if the virus is there, the test wouldn't pick it up."
Antibody tests
Antibody tests have a very different function that PCR tests, says Dr. Labus. To be tested for antibodies, you have to have blood drawn because that's where antibodies circulate in your body.
"The PCR test looks for the presence of the virus currently in your body. The antibody test looks for your immune system's response to the virus previously being there," he says. "So you have to be currently sick with the virus or infected with the virus to be positive on a PCR test, but you could be positive on an antibody test for months after you were infected."
Antibody tests are done on blood because that's where antibodies circulate in your body.
Antibody tests caused a ton of controversy back in April, because they're not all very accurate. Some brands were better than others. And unfortunately, not much has changed on that front.
"[With the] PCR test, even if it's a different brand, it's done in exactly the same way," he says. "With the antibody test, different companies are developing those kits and they're not identical, and so that's why there isn't a set [accuracy rate] for the antibody test. It really depends on which one you use." The same is true with antibody tests we use for anything. Take influenza antibody tests, for example. "There are antibody tests that can show if you were infected or vaccinated, and their ability to detect the virus really varies depending on the manufacturer."
Because it's still unknown what level of immunity those who have recovered from COVID-19 have against getting it again (there have been several cases of reinfection), anti-body tests are more effective for researchers observing patterns than individuals making decisions about their health.
Antigen tests
"The idea of an antigen test is you're looking for the outside of the virus," says Dr. Labus. "The PCR test is looking for the genetic material, the antigen test is looking for the proteins, basically the structure of that virus."
Generally, antigen tests tend to be less accurate than PCR tests, but they're a lot faster to run. Consider the rapid flu tests you can get in a doctor's office. "You can take a specimen and 15 minutes later, the doctor will have an answer. The problem, though, is you can get a lot of false results on those tests just because they're just not as accurate as PCR," he says.
And to ensure results from an antigen test are accurate, you have to run more than one. But you can't collect specimens for those tests in one sitting.
"All of those specimens are taken under the same condition. So if the virus isn't in the right place at the right time, you're going to miss it on all of those tests," he says. "Usually, if we're going to repeat an antigen test, we'll have you come back two or three days in a row so we can look at it differently. With that antigen test, basically you're trading off accuracy and speed."
Looking ahead for new COVID-19 testing methods
Having widespread testing is necessary to curb the spread of the virus. Even though testing availability continues to grow, it's not happening as fast as the incidence of the virus is increasing.
"Back in April, we had really limited capacity and we have since expanded our capacity quite a bit. The problem we had is as we expanded our capacity, we also expanded the demand for testing because a lot more people were getting sick," he says. "But as we're getting things under control in different states and changing the direction of the outbreak, we're able to test a greater percentage of the people who need it. And at some point, we'll be back to the point where we can test everybody that actually needs it."
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