Tests that detect current infections with SARS-CoV-2, the virus that causes COVID-19, are known as viral tests. There are two types: a Nucleic Acid Amplification Test, or NAAT, and an antigen test.
Many of the NAATs use a molecular biology technique known as the polymerase chain reaction, or PCR, to detect even a very tiny amount of the virus in a specimen.
The PCR test takes advantage of some natural features of biology to essentially scan through all of the RNA present in a sample — such as a nasal swab — and search for the presence of coronavirus RNA.
The Centers for Disease Control and Prevention says NAATs “are unlikely to return a false-negative result of SARS-CoV-2,” and it recommends laboratory-based NAATS, the most sensitive tests, to confirm infection.
An antigen test is designed to detect a SARS-CoV-2 protein, typically the nucleocapsid protein. Also conducted via a nasal or throat swab, antigen tests are less sensitive than NAATs but inexpensive and can usually produce results in about 15 minutes. Getting a result from a laboratory-based NAAT test can take hours or even a few days.
Testing can help stop the spread of the coronavirus from those who test positive, because they can then isolate themselves from others. But, as we’ve explained, testing has its limitations. It can take a few days or even longer after exposure to the virus before it’s detectable by the diagnostic tests. So even some with negative results could be infectious.
Still, to help limit the spread, the U.S. has instituted travel requirements and recommendations. See the CDC’s travel page for the latest guidance. International destinations may have their own testing requirements as well.
Correction, Jan. 13, 2022: An earlier version of this item misstated how an antigen test is designed to work.