Antigen tests have become a familiar term in our daily lives, particularly in the context of the COVID-19 pandemic. While PCR tests are known as the ‘gold standard’, antigen tests offer a rapid alternative for detecting active infections. But to truly understand how these tests work, it’s essential to first grasp a fundamental concept: What Is An Antigen?
In simple terms, an antigen is any substance that triggers your immune system to produce antibodies. Think of antigens as identifiers – they are molecules that the body recognizes as foreign and potentially harmful. These can be parts of bacteria, viruses, fungi, or parasites. They can also be non-living substances like toxins, chemicals, or even pollen. In the case of COVID-19, the antigen we’re concerned with comes from the SARS-CoV-2 virus, the virus that causes the disease.
Alt: Rapid COVID-19 antigen test kit for self-testing at home, showcasing components for nasal swab sample collection and result processing.
When your body encounters an antigen, it sets off a complex immune response. This response involves various types of immune cells working together to identify and neutralize the threat. One key part of this response is the production of antibodies. Antibodies are specialized proteins that are created to specifically target and bind to the antigen that triggered the immune response. This binding action can help to neutralize the antigen directly or mark it for destruction by other immune cells.
Antigen Tests: Detecting the Enemy Directly
Now, let’s connect this back to COVID-19 antigen tests. These tests are designed to detect the presence of specific viral antigens in a sample taken from your body, typically a nasal swab. Specifically, many rapid antigen tests look for the nucleocapsid protein antigen of the SARS-CoV-2 virus. This protein is a structural component of the virus.
Unlike PCR tests, which detect the virus’s genetic material (RNA), antigen tests are looking for the antigen itself – a piece of the virus. This difference is why antigen tests are often referred to as “rapid tests.” They are generally faster and less expensive than PCR tests because they don’t require the amplification steps needed to detect viral RNA. Point-of-care antigen tests can provide results in as little as 15-30 minutes.
However, this speed comes with a trade-off. Antigen tests are generally less sensitive than PCR tests. Sensitivity in this context refers to the test’s ability to correctly identify positive cases. Because antigen tests rely on detecting a sufficient amount of antigen, they may be less likely to pick up infections in the very early stages when viral loads are low, or later in the infection when the viral load may have decreased.
Alt: Chart comparing PCR and antigen tests for COVID-19, highlighting differences in sensitivity, speed, and detection method for diagnosing SARS-CoV-2 infection.
Accuracy and False Negatives
The lower sensitivity of antigen tests means that false negative results are possible. A false negative occurs when the test result is negative, but you are actually infected. This is more likely to happen if the test is performed too early or too late in the course of infection, or if the viral load is below the test’s detection limit.
Antigen tests are most accurate when there is a high pre-test probability of SARS-CoV-2 infection. This means they are most reliable when used in situations where there is a high prevalence of infection in the community, or when testing individuals who are showing symptoms of COVID-19. Viral carriage is typically highest early in infection, which means antigen tests are more likely to detect a true positive result during this period.
To improve accuracy, especially in situations where a single antigen test might not be sufficient, serial antigen testing is often recommended. Serial testing involves performing two antigen tests over a 2-3 day period, usually with 24-36 hours between tests. The FDA has authorized serial antigen tests for diagnostic purposes in people with symptoms and for screening in asymptomatic individuals, including those who have been in close contact with a confirmed case.
When Should You Use an Antigen Test?
Antigen tests are valuable tools in various situations. Here are some key scenarios where using an antigen test is appropriate, based on recommendations from health authorities:
- Symptomatic Individuals: If you are experiencing symptoms of COVID-19, especially within the first 5-7 days of symptom onset, an antigen test can provide a rapid indication of infection. This is applicable regardless of vaccination status.
- Unvaccinated Close Contacts: If you are not fully vaccinated and have been in close contact with someone with known or suspected COVID-19, testing is recommended. An initial test should be performed as soon as you become aware of the exposure, and if negative, repeat testing 5-7 days after the last known exposure, or sooner if symptoms develop.
- Fully Vaccinated Close Contacts (with Symptoms): Even if you are fully vaccinated and develop symptoms after close contact, testing 5-7 days post-exposure is advised.
- Congregate Settings: Residents and staff in settings like nursing homes or correctional facilities who develop symptoms should be tested, irrespective of vaccination status.
- Screening in Certain Situations: Antigen tests can be used for screening in specific scenarios, even in the absence of symptoms, particularly in high-risk activities or settings as advised by public health guidelines.
Reporting Your Results
It’s important to follow local guidelines for reporting your antigen test results, especially if you are using an at-home test. In many regions, there are online portals or reporting systems to help track community transmission and provide public health data.
Conclusion
Understanding what an antigen is provides crucial context for understanding how antigen tests work in the fight against COVID-19. While they may not be as sensitive as PCR tests, their speed and accessibility make them a vital tool for rapid detection, especially in specific situations and when used strategically, such as through serial testing. By detecting the presence of viral antigens, these tests play a significant role in managing and mitigating the spread of COVID-19.
