Test Quick Guide
Antigen testing is a method of detecting an active infection with SARS-CoV-2, the coronavirus that causes COVID-19 disease. These tests look for antigens, which are protein markers found on the outside of the SARS-CoV-2 virus. The test is done on a sample that is taken by rubbing the inside of the nose. The sample can be processed in a laboratory, but is frequently analyzed on-site, often providing results in about 15 minutes.
About the test
Purpose of the test
The function of antigen testing is to determine if a person has COVID-19. They are primarily used to detect and, in some cases, diagnose COVID-19:
Diagnosis is a test once a person has shown symptoms of a disease or has a known exposure to a disease. For COVID-19, molecular tests such as reverse transcription polymerase chain reaction (RT-PCR or simply PCR) are considered the gold standard for diagnosis. When those tests are not available, 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 looks for a disease in people who have no signs or symptoms. Because antigen testing can be performed quickly and at relatively low cost, it can be used in large screening programs that involve repeated testing of people to help prevent the spread of SARS-CoV-2. However, this type of screening should be done with caution due to the risk of false positive results. The risk of false positives is higher in places with low rates of virus transmission.
What does the test measure?
COVID-19 antigen tests evaluate for signs of SARS-CoV-2 infection. They check for specific proteins on the outer 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 active infection.
When should I get a COVID-19 antigen test?
There are several occasions when an antigen test for COVID-19 may be appropriate. For diagnosis, the 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 highly indicative of the disease.
Antigen tests are most often used for the detection of COVID-19. Screening programs are generally designed to prevent the spread of the virus by asymptomatic people. Some of the situations where screening can be beneficial include:
- When you have been in close contact with someone who has COVID-19
- When you have attended large social gatherings or have been in places with a high risk of transmission of the virus.
- When you 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 to travel, work, or participate in other activities
- Frequent and repeated detection with antigen tests can help identify people with COVID-19 who can then be isolated so they don’t infect other people.
Other factors that may affect when you should consider antigen testing include the likelihood of infection and the timing of possible exposure to the virus:
Likelihood of SARS-CoV-2 Infection – When you are unlikely to have been exposed to SARS-CoV-2 and the prevalence of the virus in your community is low, there is an increased risk of testing positive when you do not actually have COVID-19 , which is called a false positive.
Timing of possible exposure to SARS-CoV-2: Antigens, which are part of the virus, cannot be easily detected when the viral load is low, such as before symptoms develop, early in the disease, or when the virus is eliminated. by the body. Therefore, there is a high probability of a negative result when you are actually infected, which is known as a false negative when the test is performed during these times.
Because antigen tests can provide results quickly and cost less than PCR tests, they can be useful for screening programs. In these programs, people who test positive can be quickly isolated from others, but a second PCR or other molecular test may be needed to confirm the diagnosis.
Performing a COVID-19 antigen test
COVID-19 antigen tests require a sample of cells that come from your upper respiratory tract. In particular, they can be taken from inside the nose or from the nasopharynx, which is the upper part of the throat behind the nose. The test sample is taken with a cotton swab that is inserted into the nostril.
Before the test
Normally, you don’t need to take any special steps to prepare for an antigen test. However, you should make sure to inform the person doing the test if you have had any recent symptoms of COVID-19. For a home test, be sure to clean and disinfect your hands and surfaces in the area where you will prepare the sample.
During the exam
Whether taken in a medical or home setting, the test sample is obtained by inserting a cotton-tipped swab into the nostril. The sample can be taken from the bottom of the nose or the swab can go towards the back of the nose. Once the swab is inserted, it is usually rotated and often a sample is taken from both nostrils.
It takes less than a minute to complete the test. Inserting the swab into the nose can be uncomfortable, especially when taking a sample from the nasopharynx. You may feel a temporary sting or need to cough, and your eyes may feel watery.
After the test
There are no special post-test procedures and it is rare for it to have long-lasting effects after the test sample is taken. If a site-of-care test is to be performed, your sample will be immediately tested for viral antigens. If you are testing at home, you will need to follow the specific instructions to perform this part of the test analysis.
COVID-19 antigen test results
Receive test results
If your doctor prescribes an antigen test, your doctor may report the test results directly to you. If the sample is sent to a laboratory, it may take a few days for the test result to be obtained.
For site-of-care testing, results are typically available quickly, often within 15 minutes. When your test is done in a medical setting, you may be able to wait for the results in person or access them electronically once they are ready. With home options, results are often available via a smartphone app or displayed on your home test kit.
Interpretation of test results
The test report for an antigen test will indicate whether your sample was positive or negative for SARS-CoV-2 antigens.
Doctors interpret the antigen test result in relation to the pre-test probability of being positive or negative. If you have no symptoms, have no known exposure to SARS-CoV-2, and live in an area with low levels of transmission of the virus, your pre-test chance of testing positive is low, which means there is a higher chance that a positive result is a false positive. That said, if the test is positive, you need to isolate yourself and take steps to prevent the spread of the virus.
Similarly, if you have COVID-19 symptoms and are in close contact with an infected person, you have a high chance of testing positive before testing. In these situations, there is an increased risk that a negative result will be a false negative. Even if your test is negative, knowing the highest false negative rate, you should continue to follow CDC guidelines such as masking and social distancing.