- Experts say the current diagnostic tests for the new coronavirus are highly accurate while antibody tests are not as trustworthy.
- There are two types of common diagnostic tests — one looks for the coronavirus’ genetic material, while the antigen test searches for proteins.
- Experts say any tests must be done properly to be effective.
There’s encouraging and not so encouraging news about COVID-19 testing.
The most common tests used to diagnose an infection with the novel coronavirus are almost 100 percent effective if administered correctly.
However, the same can’t be said of tests to determine if you’ve already had the disease and have developed antibodies.
Experts say diagnostic testing is one of the most powerful public health tools for fighting the spread of the coronavirus.
The tests identify people who may need treatment. Results also trace those who have been in contact with other individuals to help prevent the transmission of the disease further. This can assist epidemiologists in determining how widely the virus has spread.
“Testing makes the enemy visible,” said Dr. Emily Volk, an assistant professor of pathology at the University of Texas-Health in San Antonio and president-elect of the College of American Pathologists (CAP).
There are two basic types of tests for the novel coronavirus. One type diagnoses an infection and the other tests for antibodies.
Two tests that diagnose an infection
Diagnostic tests detect active infections. This is the test you want if you think you’ve been exposed to the coronavirus or are exhibiting symptoms of COVID-19.
There are currently two types of diagnostic tests available.
- The molecular real-time polymerase chain reaction (RT-PCR) test detects the virus’s genetic material.
- The antigen test detects specific proteins on the surface of the virus.
The RT-PCR nasopharyngeal tests are more widely used and more familiar. Most involve sticking a 6-inch swab deep into your nose to collect virus samples to test.
However, some more recently approved RT-PCR tests seek to avoid the discomfort associated with the nasopharyngeal swab tests by allowing samples to be collected via a shallow swab of the nose or by testing saliva for the presence of the virus.
If performed correctly, RT-PCR swab tests “would be pretty close to 100 percent accurate,” Volk told Healthline.
“We should be diagnosing people with PCR tests because they are the most accurate,” added Dr. Christina Wojewoda, a pathologist at the University of Vermont and vice chair of CAP’s microbiology committee.
To get the most accurate results, RT-PCR tests should be conducted 8 days after suspected exposure or infection, to ensure that enough viral material is present to detect.
“Some clinicians know that, but people who are swabbing may not be passing that information along,” Wojewoda told Healthline.
It’s also possible to administer the test too late, after the body has successfully fought off the disease, according to Dr. William Schaffner, professor of medicine in the division of infectious diseases at the Vanderbilt University School of Medicine in Tennessee and medical director of the National Foundation for Infectious Diseases.
The test must also be administered properly, which means inserting the swab 3 inches or so to reach the cavity where the nasal passages meet the pharynx.
“If you’ve had this test and it wasn’t uncomfortable, it wasn’t done correctly,” Schaffner told Healthline.
False-positive results, while rare, can occur with PCR tests, said Wojewoda, because the coronavirus’ genetic material may linger in the body long after recovery from an infection.
“You can’t tell if the person [had an infection] 3 days ago or 5 months ago,” she said.
Swabs are also used to collect samples for antigen testing. These tests have the advantage of yielding faster results (hours rather than several days).
They’re also less accurate than RT-PRC tests, mostly because they require test samples to contain large amounts of virus proteins to yield a positive result.
False-negative results from antigen tests may range as high as 20 to 30 percent.
“If an antigen test is positive, you can believe it,” said Wojewoda. “If it’s negative, you have to question that.”
The scoop on antibody tests
As the name suggests, these tests look for antibodies made by your immune system in response to an infection with the new coronavirus.
Antibody tests are not diagnostic tests.
“Antibodies can take several days or weeks to develop after you have an infection and may stay in your blood for several weeks after recovery,” according to the Food and Drug Administration (FDA). “Because of this, antibody tests should not be used to diagnose an active coronavirus infection.”
Antibody tests also aren’t terribly useful.
Ideally, a positive antibody test would tell you that you’ve recovered from COVID-19 or a coronavirus infection and have immunity from future infections, allowing you to return to work, travel, and socialization without the risk of transmitting the infection or becoming sick again yourself.
However, researchers don’t yet know whether the presence of antibodies means that you have immunity, whether you could still get sick from a different strain of the virus, or how long immunity lasts.
“Antibody tests are problematic because they can be misused easily,” said Volk. “You may think if you have a positive antibody test that you don’t have to wear a mask or conform to social distancing, but antibodies don’t tell us that you have immunological armor against future infections.”
Antibody tests also are subject to false-positive results.
“The job of antibodies is to stick to things, so they can create a positive test result if they react to a different type of coronavirus,” said Wojewoda.
“Antibody tests show the most promise if the way the human body controls the coronavirus is with an antibody response,” Wojewoda added. “If not, it doesn’t make any difference.”
For example, she said, it’s T cells, not antibodies, that help the body fight an HIV infection.
“That’s another piece of data that needs to be figured out before testing can be figured out,” Wojewoda said.
Testing the tests
Every COVID-19 test currently (and legally) available in the United States has been approved by the FDA under the agency’s Emergency Use Authorization (EUA) protocol.
The EUA permits the FDA to “allow unapproved medical products or unapproved uses of approved medical products to be used in an emergency to diagnose, treat, or prevent serious or life threatening diseases or conditions caused by chemical, biological, radiological and nuclear threat agents when there are no adequate, approved, and available alternatives.”
That has allowed novel coronavirus tests to quickly hit the market without the research and testing normally required for FDA approval.
To date, the FDA has approved 130 different RT-PCR, antigen, and antibody tests for the new coronavirus.
“Doing a full clinical trial takes a long time, but we need tests now,” said Sherry Dunbar, PhD, senior director of global scientific affairs for Luminex Corporation, which manufactures a pair of PRC tests and has submitted an application to the FDA for emergency approval of a new antigen test.
Which test is best?
Experts generally agree that the RT-PCR tests are more accurate and useful than antigen and antibody tests, which are better used as confirmatory tools.
Dunbar told Healthline that some testing labs are using multiple tests to anticipate shortages on testing products. They’re also using the quicker tests when demand is high and the slower but more accurate tests on weekends or during slower times.
Wojewoda said that while some tests promise quicker results than others, the biggest limiting factor to turnaround results is shortages of reagents — the chemicals used to do the testing.
“I’m not looking for a new test,” she said. “Those on the market are as accurate and fast as they need to be. We have the instruments we need to test. We just need more stuff to do it with.”
As with most other things regarding the novel coronavirus, pathologists and testing labs are learning about COVID-19 on the fly, said Dunbar.
“Never in my career have I seen anything like this, where the public is discussing and analyzing the data at the same time as the researchers,” she said. “We’re basing our response on past knowledge of other viruses, but as we like to say, the bugs don’t read the book. What happened in the past can help us prepare, but things will continue to evolve.”