At-home COVID-19 rapid antigen tests have become a common tool for assessing infection status. They offer rapid results, aiding informed health decisions. As many households stocked up on these kits during the pandemic, a frequent question arises about their accuracy after the expiration date, particularly concerning false positive results.
How Rapid COVID Tests Function
Rapid COVID-19 antigen tests operate using a technology known as a lateral flow immunoassay. When a sample, typically from a nasal swab, is collected and mixed with a buffer solution, it is then applied to a small well on the test cassette. This liquid sample moves along a nitrocellulose strip through capillary action, a process where liquid flows along a narrow space without external forces.
The test strip contains specific components designed to detect viral proteins, called antigens, from the SARS-CoV-2 virus. Initially, the sample encounters a conjugate pad containing mobile antibodies linked to colored nanoparticles. If viral antigens are present in the sample, they bind to these mobile antibodies, forming antigen-antibody complexes. These complexes continue to migrate along the strip towards a “test line” where immobile antibodies are fixed.
If viral antigens are present, the complexes bind to the immobile antibodies at the test line, accumulating the colored nanoparticles and creating a visible colored line, indicating a positive result. A separate “control line” is also present, containing different antibodies that bind to excess colored nanoparticles, ensuring the test has functioned correctly and is valid.
The Purpose of Expiration Dates
Medical devices, including COVID-19 rapid antigen tests, are assigned expiration dates to ensure their performance and accuracy. They indicate the period during which the manufacturer guarantees reliable results under specified storage conditions. The components within these tests, particularly the chemical reagents like antibodies and buffer solutions, are susceptible to degradation.
Environmental factors such as temperature fluctuations, humidity, and exposure to light can cause these sensitive chemical reagents to break down. Antibodies, being proteins, can deteriorate over time, losing their ability to accurately bind to viral antigens. The liquid buffer solutions can also evaporate, potentially affecting the test’s ability to properly process the sample. Manufacturers determine these expiration dates through rigorous stability testing. Initial expiration dates were often conservative, but many have been extended by regulatory bodies like the FDA after further stability data.
Assessing False Positives from Expired Tests
False positive results from expired rapid COVID-19 tests are rare. The test’s mechanism relies on the specific binding of viral antigens to antibodies present on the test strip. For a positive line to appear, there must be a sufficient accumulation of these antigen-antibody complexes.
When test components degrade due to expiration, the antibodies typically lose their ability to bind effectively or the chemical reactions become impaired. This degradation makes it less likely for the test to generate a positive signal spontaneously or create antigens that are not present. While theoretical scenarios exist where severe reagent compromise could lead to non-specific binding, it is not a common outcome of typical degradation. Therefore, a positive result from an expired test, if it appears, is often still considered accurate, but the overall reliability of the test is compromised.
The More Common Concern: False Negatives
While false positives from expired tests are uncommon, the greater concern is the increased likelihood of false negative results. As the chemical reagents within a rapid antigen test degrade past their expiration date, their capacity to accurately detect the SARS-CoV-2 viral antigens diminishes. The antibodies on the test strip become less effective at binding to the viral proteins, even if the virus is present in the sample.
This reduction in sensitivity means that an expired test might fail to detect the virus, even in an individual who is infected with COVID-19. Obtaining a false negative result can lead to a false sense of security, potentially causing an infected individual to unknowingly spread the virus to others. This poses a public health risk, as it can hinder efforts to control transmission. Therefore, the unreliability of expired tests primarily stems from their reduced ability to correctly identify positive cases.
Recommendations for Expired Tests
Using expired COVID-19 rapid antigen tests is not recommended due to unreliable results. The chemical components can degrade over time, leading to inaccurate outcomes, particularly false negatives. Before using any test, it is important to check the expiration date printed on the box or individual test packaging.
It is also advisable to verify if the manufacturer or the U.S. Food and Drug Administration (FDA) has extended the shelf life of your specific test lot. Many tests have received extended expiration dates based on new stability data. If a test is truly expired and no extension applies, it is appropriate to dispose of it in regular household trash, as unused expired tests are typically not considered regulated medical waste. If you require testing, obtaining new, unexpired tests or seeking alternative testing methods, such as PCR tests, is the most reliable approach.