At-home COVID-19 rapid antigen tests became widely used during the pandemic, offering a convenient and accessible way to quickly check for the SARS-CoV-2 virus. These tests provide results within minutes, making them a popular choice for timely information about infection status. Their ease of use contributed to their widespread adoption for personal screening.
How Rapid COVID-19 Tests Work
Rapid antigen tests operate on the principle of a lateral flow immunoassay. When a sample, typically collected via a nasal swab, is applied, it flows along a strip containing specialized reagents.
These reagents include antibodies that are designed to bind specifically to viral proteins, known as antigens, from the SARS-CoV-2 virus. If viral antigens are present, they bind to colored particles, forming a complex.
This complex then migrates along the strip and is captured by a second set of antibodies at the test line, producing a visible colored line. A separate control line also appears, confirming the test functioned correctly.
Understanding Test Expiration
COVID-19 rapid tests carry expiration dates because their chemical components, particularly the antibodies and other reagents, can degrade over time. Manufacturers and regulatory bodies, such as the FDA, determine these dates through stability testing to ensure the test’s reliability and accuracy. The reagents can lose their effectiveness or stability when exposed to factors like temperature fluctuations, moisture, or light. This degradation means the test may not perform as intended, potentially affecting its ability to correctly detect the virus.
Expired Tests and False Positives
Using an expired rapid COVID-19 test can lead to inaccurate results, though the primary concern is often a false negative. When the chemical reagents within a test degrade, their ability to accurately bind to viral antigens can diminish. This means that even if viral proteins are present in a sample, the test might not detect them effectively, leading to a false negative result.
However, the possibility of a false positive result from an expired test is generally considered unlikely. The mechanism of action for these tests relies on specific antigen-antibody binding to produce a visible line. If the antibodies or other components have degraded, they are more likely to lose their binding capacity altogether, rather than bind non-specifically to create a false positive signal. A positive result on an expired test typically indicates that the test components were still functional, making the positive reading reliable.
Next Steps After an Expired Test Result
If an expired rapid COVID-19 test yields a positive result, it is generally considered a true positive, indicating the likely presence of the virus. However, it is recommended to reconfirm the result with a new, unexpired test. Retesting can involve another rapid antigen test or a PCR test, which is considered the gold standard for detecting viral genetic material.
Individuals who receive a positive result should consult a healthcare provider for further guidance. Following local public health guidelines for isolation and reporting is important to help prevent further spread of the virus.