How reliable are COVID-19 tests? It depends
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Testing for the coronavirus has been very much in the news. The first and most urgent focus is on increasing access to tests to diagnose people with current infections. But now other tests are appearing as well. Antibody tests, which can identify people with signs of past infection, are starting to be available. And a third type of test is on the way.
Here's a quick guide to sorting out the pluses and minuses to each type of test.
Diagnostic or PCR test
What it does: Doctors use this test to diagnose people who are currently sick with COVID-19. This is the one we've been hearing so much about.
How it works: This test uses a sample of mucus typically taken from a person's nose or throat. The test may also work on saliva — that's under investigation. It looks for the genetic material of the coronavirus. The test uses a technology called PCR (polymerase chain reaction), which greatly amplifies the viral genetic material if it is present. That material is detectable when a person is actively infected.
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How accurate is it: Generally speaking, these are the most reliable tests. However, a few days may pass before the virus starts replicating in the throat and nose, so the test won't identify someone who has recently been infected. And swabs can sometimes fail to pick up signs of active infection.
How quick is it: These samples are generally sent to centralized labs for analysis, so it can take several days to get results back. Wait times were longer earlier in the pandemic because of a testing backlog. There are also two rapid PCR tests, which can be run on specialized equipment already widely distributed throughout the U.S. The speediest one, by Abbott Laboratories, can provide a result in 13 minutes, but one study suggests this test can miss more than 10% of cases.
Antibody test
What it does: Antibody tests identify people who have previously been infected with the coronavirus. They do not show whether a person is currently infected. This is primarily a good way to track the spread of the coronavirus through a population.
How it works: This is a blood test. It looks for antibodies to the coronavirus. Your body produces antibodies in response to an infectious agent such as a virus. These antibodies generally arise after four days to more than a week after infection, so they are not used to diagnose current disease.
How accurate is it: There are more than 120 antibody tests on the market. The Food and Drug Administration has allowed them to be marketed without FDA authorization, and quality is a great concern. A few tests have voluntarily submitted to extra FDA approval. Other tests are being validated by individual medical labs or university researchers.
In general, these tests aren't reliable enough for individuals to act based on the results. And researchers say, even if you were certain you had antibodies to the coronavirus, it's still unknown if that protects you from getting sick again. Still, these tests can provide good information about rates of infection in a community, where errors in an individual result have less impact.
How quick is it: These tests generally produce results in a few minutes, based on a drop of blood taken from the finger. Some research labs use a more sophisticated antibody test, called an Elisa (Enzyme-linked immunoassay) that are more accurate but are not as widely available.
Antigen test
What it does: This test identifies people who are currently infected with the coronavirus. It may be used as a quick test to detect active infections. Initially it will not be used to diagnose disease, but it may be used to screen people to identify those who need a more definitive test.
How it works: Antigen tests can identify virus in nose and throat secretions. It does this by looking for proteins on the surface of the virus (as opposed to the diagnostic test, which looks for genetic material that is carried inside the virus). This is the same technology used in your doctor's office for rapid strep testing.
How accurate is it: These tests are not yet on the market, so there is currently no information about their accuracy. Researchers do not expect it to be as accurate as the PCR diagnostic test, but it is possible they could be used to screen patients for infection. Dr. Jordan Laser, a lab director at Northwell Health, notes antigen testing is used for rapid strep tests, which are reliable, and rapid flu tests, which are not.
How quick is it: These tests should provide results in just a few minutes. As a result, they could be used to screen people in hospitals, certain workplaces, or in other instances where it's important to find out quickly whether someone is currently at risk of spreading the disease. But unless these tests are proven to be highly accurate, physicians would still need to follow up a positive result with a PCR test to make a medical diagnosis.
You can contact NPR science correspondent Richard Harris at rharris@npr.org.
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