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: Show
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:
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:
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. Finding a COVID-19 Antigen TestHow to get testedMultiple options exist for getting an antigen test for COVID-19. Antigen tests can be prescribed by a doctor, and many antigen tests are also available without a prescription in pharmacies, health clinics, or for at-home use. In most cases, your test sample is taken and analyzed at the same location. This is known as point-of-care testing. Most point-of-care test systems are designed for rapid testing, providing results within 15 minutes. Although point-of-care antigen tests are more common, a test sample can be sent to a laboratory for analysis. For this type of testing, you may have your sample taken in a medical setting like a hospital or doctor’s office. Can I take the test at home?Kits are available for at-home COVID-19 antigen testing. Some are available with a prescription, and others are offered over-the-counter. These point-of-care tests are designed for a single use and allow you to collect a sample yourself and then analyze that sample at home. Results are usually provided within 15 minutes. How much does the test cost?Antigen tests are less costly than many other types of tests, including PCRs, but the exact cost can vary based on where the test is conducted, whether it is covered by insurance, and the specific brand of test that is used. Individual tests are usually available for less than $50 when purchased without a prescription. If an antigen test is prescribed by a doctor, its cost may be covered by insurance except for any copays or deductibles required by your plan. At-home antigen tests are available for around $25 for each individual test, although you may have to purchase packs of multiple test kits. Taking a COVID-19 Antigen TestCOVID-19 antigen tests require a sample of cells that come from your upper respiratory tract. In particular, they can be taken from inside your nose or from your nasopharynx, which is the top part of the throat that is behind the nose. The test sample is taken using a cotton swab that is inserted into your nostril. Before the testYou normally do not need to take any special steps to prepare for an antigen test. However, you should make sure to tell the person taking your test if you have had any recent symptoms of COVID-19. For an at-home test, make sure to clean and disinfect your hands and the surfaces in the area where you will prepare the sample. During the testWhether taken in a medical setting or at-home, the test sample is obtained by inserting a cotton-tipped swab into your nostril. The sample may be taken from lower in your nose, or the swab may go toward the back of your nose. Once the swab is inserted, it is usually rotated, and a sample is often taken from both nostrils. It takes under a minute to take the test. The insertion of the swab into your nose can be uncomfortable, especially when a sample is being taken from your nasopharynx. There may be a temporary sting or need to cough, and you may feel your eyes water. After the testThere are no special post-test procedures, and it is rare to have any lasting effects after your test sample is taken. If you are having a point-of-care test, your sample will be immediately analyzed to detect viral antigens. If you are taking an at-home test, you will need to follow the specific instructions for carrying out this part of the test analysis. COVID-19 Antigen Test ResultsReceiving test resultsIf your doctor prescribes an antigen test, you may be informed of your test results directly by your doctor. If the sample is sent to a laboratory, it may take a few days to get the test result. For point-of-care tests, results are normally available rapidly, often within 15 minutes. When your test is taken in a medical setting, you may be able to wait for results in-person or access them electronically once they are ready. With at-home options, the results are often available through a smartphone app or otherwise displayed on your at-home test kit. Interpreting test resultsThe test report from an antigen test will list whether your sample was positive or negative for antigens to SARS-CoV-2. Doctors interpret the result of antigen tests in relation to the pre-test probability of testing positive or negative. If you have no symptoms, no known exposure to SARS-CoV-2, and live in an area with low levels of virus transmission, your pre-test probability of testing positive is low, which means there is a greater chance that a positive result is a false positive. That being said, if you test positive, you should isolate and take measures to avoid spreading the virus. Similarly, if you have symptoms of COVID-19 and close contact with an infected person, you have an elevated pre-test likelihood of testing positive. In these situations, there is a higher risk that a negative result is a false negative. Even if you test negative, knowing the higher rate of false negatives, you must continue to follow CDC guidelines such as masking and social distancing. By comparing your pre-test probability of testing positive or negative with the actual test outcome, your doctor can determine whether a follow-up test, such as a PCR, is needed to confirm the result and diagnosis. Are test results accurate?Antigen tests are usually accurate but can return incorrect results in some circumstances.
Because of the possibility of false positives or negatives, the results of antigen tests may be confirmed with molecular tests like a PCR. The Centers for Disease Control and Prevention (CDC) has established an algorithm for health professionals to help determine when confirmatory testing is recommended. Do I need follow-up tests?Follow-up testing may be needed after an antigen test. Molecular tests like a PCR are a common way of confirming the result of the antigen test. Examples of situations when a PCR may be done after an antigen test include:
If you have clear symptoms of COVID-19, a positive antigen test may be sufficient to diagnose the disease without a follow-up PCR. Questions for your doctor about test resultsAfter you have taken an antigen test, you can review the results with your doctor and address some of the following questions:
The COVID-19 pandemic has spurred the development of different types of tests related to the condition. The table below compares and contrasts antigen tests with other kinds of COVID-19 tests: COVID-19 TestPurposeSampleAntigen TestDetect current infectionNose swabPCR and Molecular TestsDetect current infectionNose swab, throat swab, or saliva sampleAntibody TestDetect prior infectionBlood sampleSee the sections below for more detailed information on the similarities and differences between these tests. How is a COVID-19 antigen test different from a PCR or molecular test?While antigen and PCR tests both check for signs of an active SARS-CoV-2 infection, there are important differences between them:
How is a COVID-19 antigen test different from an antibody test?Antigen and antibody tests have a different purpose. Antigen tests look for active infection, but antibody tests look for past infection. Antigens are a part of the virus, while an antibody is something that the immune system creates in response to being exposed to a viral antigen. Antigen tests which look for the presence of the virus, can test for active infection, while antibodies, which develop a few weeks after viral exposure, test for past infection. Another difference is that antibody testing uses a blood sample instead of a sample taken from your upper respiratory tract. ResourcesAdditional resources that address COVID-19, including its prevention, symptoms, diagnosis, and treatment, are listed below. What does the antigen test detect?Antigen tests are immunoassays that detect the presence of a specific viral antigen, which indicates current viral infection. Antigen tests are currently authorized to be performed on nasopharyngeal, nasal swab, or saliva specimens placed directly into the assay's extraction buffer or reagent.
What is the difference between a PCR test and an antigen test?“Unlike the PCR test, the antigen test can only determine if you have an active virus in your body. The rapid test can't detect small amounts of the virus or asymptomatic cases as accurately as the PCR test can,” Heather said.
How accurate is a COVIDMolecular COVID-19 tests are generally expected to detect the SARS-CoV-2 virus at least 95% of the time when someone is infected. However, at-home COVID-19 antigen tests are generally expected to detect the SARS-CoV-2 virus at least 80% of the time when someone is infected.
|