The widespread use of at-home rapid antigen tests means many people possess kits past their printed expiration date. These over-the-counter tests detect specific viral proteins from a nasal swab sample, offering a quick way to screen for infection. Determining the reliability of an older test requires understanding how its components degrade over time. The primary concern is whether an expired test can still accurately detect the virus, impacting public health and personal safety.
The Science Behind Test Expiration
Rapid antigen tests, known as lateral flow assays, rely on precise chemical and biological components. Manufacturers set the initial expiration date based on stability studies, often using accelerated aging to estimate the shelf life under normal storage conditions. Two main parts of the kit are susceptible to degradation: the liquid buffer solution and the biological reagents on the test strip.
The buffer solution, typically in a small vial, is formulated to ensure the sample flows correctly and maintains the optimal chemical environment for detection. Over time, the buffer’s chemical composition can break down or evaporate, disrupting the flow rate and molecular binding efficiency. The biological reagents include antibodies bound to the strip and colored particles that attach to viral proteins. Since antibodies are proteins, they naturally deteriorate, losing their ability to accurately capture the SARS-CoV-2 antigen, which limits the test’s shelf life.
Reliability Risks of Using Expired Tests
The primary risk of using a genuinely expired rapid antigen test is obtaining a false negative result. Degradation of reagents or antibodies reduces the test’s sensitivity, requiring a higher amount of viral protein to trigger a positive line. If a person has low viral levels, such as during early infection or recovery, the weakened test may fail to detect the infection.
A false negative result can lead an infected person to mistakenly believe they are healthy, potentially increasing the risk of virus transmission. Degradation can also sometimes lead to an invalid result if the control line fails to appear. A false positive result is unlikely, as component degradation decreases the likelihood of the necessary chemical reaction.
Regulatory Shelf Life Extensions
The expiration date printed on the test box is often an initial, conservative estimate, especially for tests authorized early in the pandemic. Regulatory bodies, such as the Food and Drug Administration (FDA), established a process to grant shelf life extensions after the initial date passes. Manufacturers perform real-time stability testing beyond the initial authorization period, monitoring the test’s performance over longer durations.
When this data demonstrates the components remain stable and accurate, the regulatory body approves an extension. This means a test with a past printed expiration date may still be valid for use. To verify the updated shelf life for a specific kit, users should refer to the regulator’s official website, using the test brand and the lot number found on the box.
Maximizing Accuracy and Next Steps
Proper storage is necessary for maintaining accuracy up to the stated expiration date. Most tests are temperature-sensitive and should be stored within the range specified on the packaging, typically at room temperature away from excessive heat or cold. Exposure to high temperatures, such as leaving a kit in a hot car, accelerates the degradation of chemical components and antibodies.
If using a test truly past its extended expiration date, check the liquid buffer for signs of degradation like cloudiness, discoloration, or reduced volume. If the result is negative but symptoms persist, it is advisable to take a new, unexpired test or seek a more sensitive molecular test for confirmation. Following the detailed instructions precisely, including the waiting time, helps ensure the most reliable result.