Purpose of the test
The role of antigen tests is to determine if a person has COVID-19. They are primarily used in screening for and in some cases diagnosing COVID-19:
Diagnosis is testing once a person has shown symptoms of a disease or has a known exposure to a disease. For COVID-19, molecular tests like the reverse transcription polymerase chain reaction (RT-PCR or just PCR) are considered the gold standard for diagnosis. When those tests are unavailable, an antigen test can be used for diagnosis in people who have symptoms consistent with COVID-19. However, antigen tests have a higher risk of indicating a negative result in someone who has actually been infected with the virus, also called a false negative result.
Screening is looking for a disease in people who don’t have any signs or symptoms. Because antigen tests can be performed rapidly and at relatively low cost, they may be used in large screening programs that involve repeatedly testing people to help prevent the spread of SARS-CoV-2. This type of screening must be done cautiously, though, because of a risk of false positive results. The risk of false positives is highest in places with low rates of virus transmission.
What does the test measure?
COVID-19 antigen tests evaluate whether there are indications of infection with SARS-CoV-2. They check for specific proteins on the outside surface of the virus called antigens in a test sample taken from the nostril or the area at the back of the nose. The presence of these antigens is a sign of an active infection.
When should I get a COVID-19 antigen test?
There are various times when an antigen test for COVID-19 may be appropriate.
For diagnosis, an antigen test is most often used when PCR tests are not readily available. In these situations, a positive antigen test can be used to diagnose COVID-19 if your symptoms are strongly indicative of the disease.
Antigen tests are more often used for screening for COVID-19. Screening programs are typically designed to prevent the spread of the virus by people who are asymptomatic. Some of the situations in which screening may be beneficial include:
- When you have been in close contact with someone who has COVID-19
- When you have attended large social gatherings or otherwise been in places at high risk for virus transmission
- When you either live or work in places where many people live together such as nursing homes, homeless shelters, or correctional facilities
- When you need proof of a negative test in order to travel, work, or engage in other activities
Frequent and repeat screening with antigen tests may help identify people with COVID-19 who can then be isolated so that they do not infect other people.
Other factors that can affect when you should consider antigen testing include the likelihood of infection and timing of possible virus exposure:
- Likelihood of SARS-CoV-2 infection: When it is unlikely that you have been exposed to SARS-CoV-2 and the prevalence of the virus in your community is low, there is a greater risk of testing positive when you don’t actually have COVID-19, which is called a false positive.
- Timing of potential SARS-CoV-2 exposure: Antigens, which are a part of the virus, can’t easily be detected when the viral load is low, such as before symptoms develop, in early disease stage, or as the virus is cleared by the body. Therefore, there is a high chance of a negative result when you are actually infected, which is known as a false negative when tested during these times.
Because antigen tests can deliver results quickly and cost less than PCR tests, they can be useful for screening programs. In these programs, people who test positive can be promptly isolated from others, but a second test with a PCR or other molecular test may be needed to confirm the diagnosis.