At-home COVID-19 tests offer convenience and rapid results for assessing infection status. A frequent concern is their reliability, especially when they approach or pass their expiration dates. Understanding how these tests work and how time affects their components clarifies the accuracy of results from expired kits.
How At-Home COVID Tests Function and Expiration’s Role
Rapid antigen tests detect specific viral proteins, known as antigens, in a nasal or saliva sample using a lateral flow immunoassay. The test strip contains antibodies linked to colored particles that bind to these viral antigens. As the sample flows, any viral antigens present form a complex with these colored antibodies. A test line then captures these complexes, creating a visible line if the virus is present. A separate control line confirms the test functioned correctly.
Expiration dates are set because the chemical reagents and biological elements, such as antibodies, degrade over time. This degradation reduces their stability and effectiveness, impacting the test’s ability to accurately detect the virus or even function as intended.
Can Expired Tests Lead to False Positives?
The question of whether an expired COVID test can yield a false positive result is often raised. While experts suggest false positives from expired tests are generally unlikely, the theoretical possibility exists due to the degradation of test components. When chemical reagents and antibodies degrade, they might lose specific binding capabilities. This could, in some scenarios, lead to non-specific binding or chemical reactions that mimic a positive result.
If control line components degrade, it could also interfere with proper test function. However, if a positive result appears on an expired test, it is often still accurate, suggesting the antibodies are detecting the virus. The overall consensus leans towards false negatives being a more prevalent issue with expired tests than false positives.
The Risk of False Negatives from Expired Tests
A more significant concern with using expired COVID-19 tests is the increased risk of obtaining a false negative result. As antibodies and reagents degrade past their expiration date, their ability to accurately capture and react with viral antigens diminishes. This reduced sensitivity means the test might not detect the SARS-CoV-2 virus, even if a person has a detectable viral load.
If the chemicals break down, the necessary chemical reaction to display a positive result might not occur, leading to an inaccurate “negative” reading. This decreased reliability can have serious public health implications, as an individual unknowingly infected might mistakenly believe they are negative and inadvertently spread the infection. Relying on an expired test for a negative result can create a false sense of security.
Interpreting Your Results Safely
Given the potential for inaccurate results, exercising caution when interpreting outcomes from any at-home COVID-19 test, especially one past its printed expiration date, is important. First, check the FDA’s website, as many tests have received extended expiration dates based on further stability studies. If your test’s lot number indicates an extended shelf life, it may still be reliable.
If you use an expired test and receive a negative result, particularly with symptoms or a known exposure, retest with an unexpired kit within 48 hours. For greater accuracy, a confirmatory PCR test can be considered, as these tests are generally more sensitive. Consulting a healthcare professional is always a prudent step for guidance on testing, interpreting results, and making informed decisions about your health and public safety.