Antigen tests can identify the presence of antigens within the body, which are substances that cause immune responses, such as the release of antibodies.
The immune system, and antibodies specifically, recognize the antigens. Rapid antigen detection tests are based on this principle. By now, many people have experienced an antigen test, a polymerase chain reaction (PCR) test, or both for COVID-19. These tests typically involve a swab of the nose or throat.
In this article, we look at what an antigen test is, how it works, and the benefits and risks of this type of test. We also compare antigen tests with PCR tests.
An antigen is a substance that can trigger an immune response when present in the body. It may be a virus, bacterium, toxin, chemical, or other substance from outside of the body. Tissues and cells in the body also contain antigens that can cause immune responses.
If a sample from a person’s nose or throat contains the virus, the antigen test will indicate the result with a visual change. In an over-the-counter (OTC) antigen test, a positive result typically appears as a line on the test, similar to a pregnancy test.
Antigen tests have the advantage of simplicity and speed. An antigen test can quickly identify easy-to-find surface markers on the outside of the virus. A person can collect samples easily by swabbing the nose or throat, where the virus tends to replicate and gather in large numbers.
Antigen tests can also produce results within minutes. That said, they may not be as accurate as PCR tests, which are the other common form of testing used for COVID-19.
For an antigen test, a person will take a swab sample from their nose and throat and put it into a liquid containing soap and salt that breaks apart particles and cells. They will then apply the liquid to a test strip containing antibodies specific to the virus. Antibodies on the strip will bind to antigens in the sample, just as they would inside the body.
A visible line will indicate the presence of the virus or other antigen in a positive sample.
Antigen tests have various benefits, including:
The major risk of an antigen test is that it can be less accurate than other types of tests.
If a person is symptomatic and has a large amount of virus in their system, an antigen test is typically very accurate. The problem is that antigen tests are not as sensitive as PCR tests because antigen tests do not amplify the antigens or virus they are testing for, unlike PCR tests.
This means if a person is in the early stage of infection, and there is not yet a large number of antigens in the area being swabbed, the test can provide a false-negative result.
One analysis of 133 studies evaluating the accuracy of 61 different antigen tests for the detection of SARS-CoV-2 found mixed results.
The researchers found that antigen tests were much more accurate in symptomatic patients, at 76.7%, than in asymptomatic patients, at 52.5%. This leaves a lot of room for error.
The analysis also found that over half of the studies analyzed did not perform the antigen tests in accordance with test manufacturers’ recommendations. This is another risk of antigen tests. While they may not require a professionally qualified person to perform the test or interpret the results, they are open to user error, which is likely to affect their accuracy.
PCR tests differ from antigen tests, as they do not simply identify antigens in a sample.
For a PCR test, a person must amplify the genetic material in a sample, which allows the test to detect even a small amount of the virus. To do this, they use a technique called polymerase chain reaction, which involves treating the sample with an enzyme to convert viral ribonucleic acid (RNA) into double-stranded DNA. Through further processing, the virus DNA can be amplified.
Each method of testing has advantages and disadvantages.
|Antigen test||PCR test|
|Swab or specimen type||nasal or nasopharyngeal (upper throat)||nasal, nasopharyngeal, saliva, sputum, oropharyngeal (back of the throat)|
|Component detected||viral antigens||viral ribonucleic acid (RNA)|
|Accuracy||moderate to high when viral load is at its peak||varies by test, but generally high for laboratory-based tests and moderate to high for point of care (POC) tests|
|Difficulty||relatively easy||varies by test|
|Cost||low, between ||moderate, between|
|Advantages||• short turnaround time|
• rapid identification of virus, which could prevent further spread
|• most sensitive testing method available|
• usually does not require a person to repeat the test to confirm results
|Disadvantages||• may need follow-up testing to confirm results|
• less sensitive, which could produce false results
|• longer turnaround time|
• higher cost
An antigen test detects antigens in the body that cause an immune response, such as the release of antibodies. Antigen tests for COVID-19 detect proteins of the SARS-CoV-2 virus.
To perform a test, a person takes a swab sample. Then, the test determines whether antibodies bind to antigens in the sample. If they do, the test will respond by changing in color or fluorescence to indicate a positive result.
Antigen tests are beneficial because they have a fast turnaround time, are easy to perform and interpret, and are cost-effective. However, they may not be as accurate as PCR tests, especially in the beginning stages of a virus when the viral load has not gathered in large amounts yet.
PCR tests are more sensitive and accurate, but they are more difficult to perform, have a slower turnaround time, and are costlier.