Health Library

Two kinds of viral tests can see if someone is infected with coronavirus: molecular tests (such as PCR) and antigen tests. Because they help to diagnose infection, they’re also called “diagnostic tests.”

• Molecular tests are very sensitive, which means they can find even very small amounts of the virus. But it can take time for the results to be ready. 
• Antigen tests are a little less sensitive and can sometimes miss the virus when someone has low amounts of it in their body at the start or end of the illness. Test results, though, can be ready very quickly, which is why they’re often called “rapid” tests.

Note: A different test (an antibody test) can see if someone was infected in the past or is vaccinated.

There isn’t one test that’s best for all situations. Which one someone needs can depend on:

• how quickly they need the results
• how important it is to get the most accurate result
• how easy or convenient it is to get the test

It can be tricky to figure out which test to get. Your doctor can help you figure out which test you or your child might need.

When Someone Has Symptoms of COVID-19

Both types of tests can help to see if symptoms are due to COVID-19:

• Molecular tests are so sensitive that they can find even a small amount of virus in a person’s nose or throat. These tests are the best option when someone needs to know for sure if they’re infected (to get treatment, for example). But it can take at least a day or two to get the results.
• When quicker results are needed, an antigen test can be a good option. For example, parents might need to know right away if a child is infected so they can isolate them and prevent spread in the household. When they’re not at the start or end of the illness, people who are infected have a lot of virus in their nose and throat. This is when they’re most contagious, and when antigen tests can be as sensitive as molecular tests. If an antigen test does not show infection, a doctor may choose to confirm the negative result with a molecular test.

When Someone Has No Symptoms After Exposure to COVID-19

Both types of tests can help check whether someone has the virus after being exposed to an infected person:

• Because molecular tests are more sensitive, they can detect infection even when there’s very little virus in the nose or throat. This makes them more accurate for people who don’t have symptoms, as they tend to have less virus in their nose and throat than people who feel sick.
• If quick results are needed, an antigen test is good too. If it doesn’t show infection on the first try, doing the test again a day or two later often can catch the virus as levels rise. People who are vaccinated should get tested 5–7 days after exposure. People who aren’t vaccinated should get tested as soon as they know they’ve been exposed, and again 5–7 days after exposure.

When Schools and Workplaces Routinely Screen

Checking people with no symptoms and no known exposures is called screening. Often used in schools or workplaces, screening is an important way to slow the spread of infection. Both types of tests can be used for screening, but those with quick results are the most practical. So, when schools and workplaces screen, they generally do antigen tests. They’re quick, easy to use, and less expensive. They’re less sensitive than molecular tests, so often need to be repeated (such as 2–3 times per week).

The decision about whether a school or workplace should screen depends on the rate of COVID-19 infections in the community. Schools might screen student groups that are at higher risk for infection, such as athletes, band or choir members, or theater groups. Or they might choose to screen before large events like a prom, or after students return from breaks.

When People Plan to Gather

Holiday and other gatherings often involve people from different households getting together indoors and removing masks to eat and drink. Rapid testing right before the event can help keep everyone safe and healthy. If all guests have a negative antigen test before an event, the chances are very low that someone is contagious and could spread coronavirus.

When deciding whether to test, consider the rate of COVID-19 infections in your community and whether anyone in your group has a higher risk of getting very sick with COVID-19 (such as young infants, elderly adults, or people with certain health conditions or weak immune systems).