At-home COVID-19 antigen tests offer a quick and convenient way to check for the virus. When people use an old kit, they often find the printed expiration date has passed. This raises a common question: Can an expired test create an inaccurate positive result? Understanding the science behind these tests clarifies how their performance changes once they are past their labeled shelf life.
Why Do COVID Tests Expire?
The expiration date relates to the stability of the test’s chemical and biological components. Rapid tests use specific antibodies embedded in the strip to capture and bind to SARS-CoV-2 viral proteins (antigens). This binding triggers a visible color change indicating a positive result.
These antibodies are delicate protein molecules that degrade over time, a process accelerated by environmental factors like temperature fluctuations or high humidity. As these components deteriorate, the test’s ability to reliably perform the chemical reaction is compromised, necessitating a shelf life.
The Likelihood of a False Positive Result
The core mechanism of how an expired test fails makes a false positive result highly improbable. A false positive occurs when the test indicates the presence of the viral antigen even though the person is not infected, usually due to cross-reaction or contamination.
When the antibodies in the test strip degrade, they lose their ability to bind effectively to the viral protein. This means they are less likely to produce the color-change signal. The degradation of components results in a weaker or absent reaction, not the spontaneous creation of a positive line. If an expired test shows a positive result, it is generally considered a true positive because the antibodies were still functional enough to detect the viral antigen.
The Greater Risk: False Negatives and Inconclusive Results
The far more significant risk associated with using an expired antigen test is obtaining a false negative result. As the antibodies and other reagents break down, the test’s sensitivity decreases, meaning it requires a higher concentration of viral particles to trigger a positive reading. An infected person with a low viral load might test negative because the degraded test is no longer sensitive enough to detect it.
This outcome creates a false sense of security, potentially increasing the risk of virus transmission. Degradation of other components, such as the liquid buffer solution, can also lead to invalid or inconclusive results. The buffer may evaporate or change composition, which prevents the proper flow of the sample across the test strip. This often leads to the control line failing to appear, indicating the entire test failed.
Official Guidance on Using Expired Test Kits
Consumers with older test kits should first check for authorized shelf-life extensions. When tests were initially authorized, manufacturers used conservative expiration dates based on initial stability testing. Ongoing studies have since shown that many tests remain stable and reliable for much longer periods, sometimes up to two years.
The U.S. Food and Drug Administration (FDA) maintains a public list where consumers can check if the expiration date for their specific test brand and lot number has been extended. If the lot number is listed with an extension, the kit can be used confidently until the new date. If the test is truly past its extended expiration date, regulatory bodies recommend acquiring a new, non-expired kit to ensure accurate results.