At-home rapid antigen tests have become a common tool for quickly assessing the risk of a COVID-19 infection. These tests offer convenience and fast results, but they also come with a printed expiration date that can cause confusion. Determining whether a test kit is still viable past the date stamped on the box requires understanding the biological materials within the kit, the regulatory process for shelf life, and the potential impact on the accuracy of the result.
The Science Behind Test Expiration
The expiration date reflects the stability of the test’s biological and chemical components over time. Most tests rely on a lateral flow assay, which uses specialized materials to capture viral proteins from a nasal swab sample. The main components at risk of degradation are the test strip’s embedded antibodies and the liquid reagent solution.
Antibodies are proteins designed to specifically bind to the SARS-CoV-2 antigen. Over time, these proteins can deteriorate and lose their ability to bind effectively. When antibodies degrade, the test’s sensitivity is reduced, meaning it may struggle to detect the virus even if a person is infected.
The liquid solution can also be affected by evaporation or chemical breakdown, compromising the necessary environment for the reaction to occur. The expiration date represents the final point at which the manufacturer ensures the test performs with the same high level of accuracy as when it was first produced.
Official Guidance on Extended Shelf Life
The expiration date printed on the packaging is based on accelerated stability studies conducted by the manufacturer. When tests were first authorized, limited time was available to study long-term viability, resulting in shorter initial shelf lives. Manufacturers continue to conduct additional stability testing and submit that data to regulatory bodies.
If further testing demonstrates the test remains accurate beyond the original date, the regulatory body may authorize an extension of the shelf life. This means a test with a passed printed expiration date may still be valid under the new, extended date. The authorized extension date always supersedes the date printed on the box, especially since packaging may have been printed before new stability data was available.
To determine if a specific kit is still authorized, individuals must check the official list of authorized at-home tests maintained by the regulatory body. This requires locating the test’s brand name and the specific lot number, often found on the box. If the test and lot number appear on the list with an extended date, it can be used until that new expiration date. If the information is not on the list, or if the extended date has passed, the kit should be considered truly expired.
Reliability and Interpretation of Results
The primary concern with using a truly expired rapid antigen test is the significant risk of a false negative result. Because the core components, like the virus-capturing antibodies, have degraded, the test may not be sensitive enough to detect viral antigens in the sample. This failure to detect the virus can occur even when a person is actively infected and potentially contagious.
A false negative result can lead a person to mistakenly believe they are not infected and fail to take appropriate isolation measures. Conversely, the risk of an expired test producing a false positive result is low. Degradation typically affects the test’s ability to react, not its specificity, meaning a positive result from an expired test is highly likely to be accurate.
If a negative result is obtained from a test past its authorized extended shelf life, it should not be trusted, especially if symptoms are present. The most reliable course of action is to retest with a new, non-expired rapid antigen test within a day or two. Alternatively, a laboratory-based molecular test, such as a PCR test, can be sought for definitive confirmation.