At-home COVID-19 rapid antigen tests became a widely available tool for individuals to quickly check for the presence of the SARS-CoV-2 virus, offering results within minutes. Their accessibility made them a common household item, playing a part in managing personal health and community spread. A frequent question arises regarding their effectiveness once the printed expiration date has passed, specifically concerning the possibility of a false positive result. Understanding the limitations of these tests over time is important for accurate interpretation.
Accuracy of Expired Tests
Using a rapid antigen test past its expiration date compromises its reliability. The primary concern with expired tests is an increased risk of false negative results, meaning the test fails to detect the virus even if it is present. This can lead to unintended viral spread.
False positive results, suggesting an infection when none exists, are considered less common with expired tests than false negatives, because a degraded test is less likely to react falsely to non-viral components. However, any result from a test past its stable shelf life should be viewed with skepticism, as the test’s ability to accurately detect viral proteins is no longer guaranteed. Some studies suggest that expired tests might still show a positive result if a strong viral load is present, but their sensitivity decreases.
How Tests Degrade Over Time
Rapid antigen tests have expiration dates because their components, particularly the chemical reagents, degrade over time. These tests rely on sensitive antibodies designed to bind to specific viral proteins of SARS-CoV-2. Over months, these protein-based antibodies can break down, become less reactive, or dry out, impairing their ability to accurately capture the viral antigens in a sample.
The liquid solution included in the test kit, which helps process the sample, can also evaporate, affecting the test’s proper functioning. This degradation can lead to a reduced ability of the test to detect the virus, resulting in either a false negative or an invalid result. The control line, which confirms the test ran correctly, can also be affected by degradation, further complicating result interpretation.
What to Do With Expired Test Results
It is generally recommended to avoid using COVID-19 tests that have passed their expiration date. However, first verify if the expiration date on your test has been extended by the manufacturer and the U.S. Food and Drug Administration (FDA). Manufacturers often conduct further stability testing, and the FDA may approve extended shelf lives for specific test lots, sometimes for many months beyond the date printed on the box. If an expired test has been used, especially if symptoms are present or exposure has occurred, obtaining and using a new, non-expired test is advisable for a more reliable result. Following current public health guidelines regarding isolation and masking, regardless of an expired test result, remains a prudent approach when experiencing symptoms or after exposure.