Antigen versus antibody testing for COVID-19: What you need to know

As federal officials gradually begin reopening the country, expanded testing is central to the conversation. Until now, there were two types of commercially available tests: PCR-based tests and antibody tests. But on Friday, the Food and Drug Administration authorized use of the first antigen test -- a potentially cheaper and faster way to detect people who have the virus.

PCR-based tests look for the virus's genetic material in saliva or mucus samples to diagnose people who are currently infected with the novel coronavirus. Antibody tests use blood samples to find signs of prior viral infection.

But in a recent briefing, Dr. Deborah Birx, the White House coronavirus response coordinator, explained that the available tests won't help get Americans back to work in a meaningful way.

"There will never be the ability on a nucleic acid test to do 300 million tests a day or to test everybody before they go to work or to school," she said. "But there might be with the antigen test."

An accurate, FDA-authorized antigen test could be a "breakthrough" innovation to help screen more people for the virus, according to Birx.

But some experts are still skeptical that a reliable antigen test could even be developed.

Here's what you need to know about each of these tests.

Antigen tests

In theory, antigen tests can be used to screen individuals infected with the virus, but these tests did not become FDA authorized until May 8. Emergency use authorization was granted late Friday to the Quidel Corporation for the Sofia 2 SARS Antigen FIA.

"These tests are less accurate than antibody and PCR tests, but they are great for detecting new cases," according to Dr. Simone Wildes, an ABC News Medical contributor and an infectious diseases physician at South Shore Health in Massachusetts.

Antigen tests use a nasal or throat swab to detect viral proteins expressed on the outer surface of the coronavirus. Antigens, a term for any foreign substance, like a virus or bacteria, are recognized by the body to induce an immune response to fight off infection.

These inexpensive tests could potentially deliver results within 30 minutes. In a recent scientific briefing, the World Health Organization cautioned again the use of rapid antigen tests because the science is still so new and the tests are unreliable. Further research needs to be done, the WHO said.

Only a few companies are developing COVID-19 antigen tests. OraSure Technologies, Inc., a biomedical company that develops technologies for testing and diagnosing medical conditions, is one company that is working to create a COVID-19 antigen test that could potentially deliver results in 20 minutes using an oral fluid sample.

"OraSure's proposed rapid test would be designed for in-home self-testing by lay users, though medical professionals will be able to use it in clinical settings too," Stephen Tang, president and CEO of OraSure Technologies, Inc., said in a statement to ABC News.

Antibody tests

Antibody tests differ from antigen tests in that they detect the presence of proteins produced by the body in response to a previous infection.

Antibodies are produced by the immune system -- the body's defense against sickness -- to fight against viruses and other harmful pathogens that enter the body, Wildes said.

"An antibody test measures antibodies in the blood to determine if you have had previous infection against the virus and have recovered," she added.

While these tests cannot be used to diagnose infections, they can show whether or not an individual was infected in the past and can help track the spread of an infectious disease. Scientists are still trying to understand whether or not having antibodies will protect someone from future reinfection, and if so, for how long.

Typically administered by a health care professional, antibody tests require a sample of blood. These tests help identify patients who have antibodies in their blood and would like to donate their plasma as part of an experimental COVID-19 treatment for those who are critically ill or need help with recovery.

The FDA, to date, has issued emergency authorizations to 12 antibody tests for the novel coronavirus. On Sunday, the FDA cleared an emergency use authorization for an antibody test from Roche Holding AG, a Swiss health care diagnostics giant. Over 200 more antibody tests are currently in the process of review to receive emergency use authorization from the FDA.

Across the nation, more testing sites have a variety of COVID-19 tests available for people who are currently infected or suspect they may have had the virus in the past. CityMD Urgent Care Walk-in Medical Clinic recently announced that it will be providing RT-PCR and antibody testing at all of its locations nationwide. CareWell Urgent Care centers throughout Massachusetts are testing for antibodies. StatCare Urgent and Medical Walk-in Care centers are providing RT-PCR and antibody testing throughout New York.

Each test for COVID-19 has its promises and shortcomings. As we learn more about the virus, our testing methods will improve, and vice versa. Widespread testing, when used hand-in-hand with other methods to slow the virus, will help stop this pandemic. Will antigen tests play a role in stopping the virus? Only time will tell.