The difference between diagnostic, antibody, and antigen tests for the coronavirus

• Diagnostic or polymerase chain reaction (PCR) tests are currently being used to diagnose patients with COVID-19.
  
  
• Antibody tests allow for more accurate tracking of the spread of the coronavirus. People who test positive for coronavirus antibodies can also donate plasma.
• Antigen testing is not on the market yet, but Massachusetts-based E25Bio is among several companies seeking FDA approval for at-home test kits.
• Visit Business Insider's homepage for more stories.

With new information released all the time, it can be difficult to keep track of how doctors are testing for the coronavirus.&

While identifying and treating infected patients is critical, some tests add to our greater understanding of the pandemic's size, impact, and direction. Here is a breakdown of the differences between diagnostic, antibody, and antigen testing.

Diagnostic testing: the frontline defense

A diagnostic or a polymerase chain reaction (PCR) test is used for someone suspected of being infected with the coronavirus, usually obtained through a nasal or throat swab.

Due to the limited supply in the US, though, typically only people with severe symptoms are tested.

"This test is looking for the presence of viral genetic material indicating the person has a current infection and is likely infectious," epidemiologist Tista Ghosh told Insider.

"Pathogens can be seen directly in specimens from the patient, often using a microscope, or grown in the laboratory over a period of days to weeks," Stephen Berger, founder of the Global Infectious Diseases and Epidemiology Network, said.

Nucleic acid tests, which are more sensitive in early detection, have been used extensively during the current pandemic. Results can typically take at least one to two days, though more rapid versions are in development.

While they can detect the virus in those who are asymptomatic, diagnostic tests present a false negative in about 30% of cases. Patients are therefore tested twice before being confirmed as noninfectious.

Antibody testing: Are you immune?

People who may have been infected and recovered are checked for antibodies, proteins the body creates to help fight off a particular infection.

Coronavirus antibodies are generally not detectable until 14 days after symptoms began.

"In most viral infections, specific antibodies can be detected for months to years following infection — and in many cases well into old age," says Berger.

Once someone is confirmed to have the antibodies they're able to donate plasma to patients who are currently infected. But antibody testing is also used to gain a more accurate representation of the pandemic's overall impact.

Widespread antibody testing can help track where the virus has spread and its fatality rate, and determine whether or not it's appropriate to ease a lockdown.

Unlike the swab for diagnosis, antibody tests require a blood sample, usually through a finger prick.

The blood is then examined for two types of antibodies: IgM antibodies, which develop early during an infection, and IgG antibodies, which are more likely to be detected after you've recovered and are a better indicator of at least partial immunity.

IgM test results can be available in as little as 15 minutes, while IgG tests are sent away to a lab and it can take up to a week for results. But some clinics will only perform IgG blood tests because they're more reliable.

Does having antibodies to the coronavirus mean you're immune? It's not clear.

"In theory, the presence of these antibodies indicates a person has a low likelihood of being infected again," Ghosh said. "But we do not yet know the degree of immunity or how long this immunity lasts."

It's also not clear if antibody tests cross-react with antibodies of other, more common coronaviruses or if immunity provided by antibodies from one strain of the coronavirus extends to another strain.

And a negative result doesn't necessarily mean you didn't have COVID-19. It could mean you have an active infection and just haven't built up any antibodies. Even without symptoms, you'd still be at risk for spreading the virus.

The FDA has issued emergency use authorizations to increase antibody testing for the. Currently, there are over 120 different tests on the market, but not all are effective.

"Early results suggest that many of the serology tests on the market today are inaccurate and unreliable, and may have a high rate of false positives," Ghosh said. "So employers will need to carefully vet the quality of a given serology test before selecting a vendor, and will need to test employees multiple times to reduce the rate of false readings."

On Monday, the FDA announced that companies have 10 days to prove the accuracy of their antibody tests or risk removal from the market.

Antigen testing: the path to reopening America

While PCR tests pick up the coronavirus' genetic material, antigen tests register surface proteins on the virus. They're easier to spot, making testing quicker and cheaper, and potentially available for at-home use.

"The presence of COVID-19 and many other viruses in patient specimens can be confirmed using these antigen tests, usually within minutes to hours," Berger said. "In COVID-19, the viral antigen will already be detectable at the first sign of disease. In fact, many patients will already excrete viral RNA for several days before there are any symptoms whatsoever."

An antigen can only indicate if you are currently infected, not if you recovered from the virus.

White House coronavirus response coordinator Deborah Birx indicated at a recent briefing that reliable antigen testing could help reopen America, ABC News reported.

"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," Birx said. "But there might be with the antigen test."

There are no antigen tests currently on the market, but several companies are working on them. OraSure Technologies is developing a home kit that would allow users to test themselves with a saliva sample.

E25Bio, backed by MIT scientists, is seeking FDA approval for a rapid-response kit that could provide results within 15 minutes.

E25Bio's test resembles an over-the-counter pregnancy test: A nasal swab is taken, with the sample added to a solution and placed on a paper strip treated with artificial antibodies. If the test is positive for coronavirus, a colored indicator forms on the strip.

While antigen tests ease and speed make them attractive, their lack of sensitivity does not: They've been found to miss about 15% to 20% of infections.

"If you're using this test to screen people to make sure that they're not infected, and that they go back to work or things like that, then you'd be giving people the message that they are not infected when actually they are — and therefore transmitting the new coronavirus," Jesse Papenburg, an infectious disease expert at McGill University, told NPR.

For an antigen test for coronavirus to really work, scientists must understand the biology and structure of the coronavirus, something they're only beginning to do.

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