About the Test
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.
Finding a COVID-19 Antigen Test
How to get tested
Multiple 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 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 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 test
You 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 test
Whether 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 test
There 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 Results
Receiving test results
If 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 results
The 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.
- Accuracy can be affected by incorrect sample collection or contamination that occurs when taking or processing a sample. For this reason, carefully following test instructions is essential.
- False positive results can show the presence of SARS-CoV-2 when no virus is actually present. The risk of false positives is highest when a person has limited potential contact with the virus, such as in areas with low levels of community transmission of SARS-CoV-2.
- False negative results can show no signs of SARS-CoV-2 when a person is actually infected. This most frequently occurs when a person is tested too soon or too late after being infected.
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 the test is positive, especially if you are asymptomatic, have not had close contact with anyone with COVID-19, and live in an area with limited cases of the disease.
- If the test is negative when you have symptoms that are indicative of COVID-19.
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 results
After you have taken an antigen test, you can review the results with your doctor and address some of the following questions:
- Is it possible that my test result was inaccurate?
- Do I need another test? If so, what kind?
- Do I need to take any special steps or precautions as a result of my test result?
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 Test | Purpose | Sample |
|---|---|---|
| Antigen Test | Detect current infection | Nose swab |
| PCR and Molecular Tests | Detect current infection | Nose swab, throat swab, or saliva sample |
| Antibody Test | Detect prior infection | Blood sample |
See 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:
- A PCR or molecular test looks for traces of the virus’s genetic material instead of proteins found on the outside of the virus.
- A PCR can be conducted on a sample taken from your nose, throat, or saliva, but antigen tests are currently only performed on samples from your nose or behind the nose.
- While both are available as both laboratory and point-of-care tests, antigen tests are more frequently used for point-of-care testing.
- Antigen tests generally cost less than PCR tests.
- PCR tests are generally considered to be more accurate and may be used to confirm the results of an antigen test.
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.
Resources
Additional resources that address COVID-19, including its prevention, symptoms, diagnosis, and treatment, are listed below.
