The question of whether an expired COVID-19 test can produce a false positive result is a common concern. Experts agree that while an expired test may yield an inaccurate result, the primary risk is a false negative, not a false positive. Using a test past its stable shelf life significantly increases the chance of receiving a misleading negative result. The mechanisms of chemical degradation make it far more likely that the test will simply fail to detect the virus rather than mistakenly indicate its presence.
How Antigen Tests Function
The at-home COVID-19 test uses a technology known as a lateral flow assay. This simple, paper-based system is designed to detect specific viral proteins, or antigens, from a nasal swab sample. The collected sample is mixed into a liquid buffer solution and then applied to the test strip’s sample pad. The liquid then travels along the strip through capillary action, carrying any viral proteins with it.
As the liquid moves, it passes through a reagent pad containing specialized detection antibodies. These antibodies are chemically linked to tiny, colored nanoparticles, often colloidal gold. If the SARS-CoV-2 antigen is present, it binds to these colored antibody-nanoparticle complexes. This entire complex then continues its journey toward the test lines.
The strip contains two distinct lines: the Test (T) line and the Control (C) line. The T-line is coated with capture antibodies designed to grab the antigen-nanoparticle complex, causing the colored nanoparticles to accumulate and form a visible line. The C-line captures any remaining mobile nanoparticles, confirming that the liquid successfully flowed across the strip and that the test functioned correctly.
The Chemical Reasons for Expiration
The expiration date is tied directly to the stability of the test’s biological and chemical components. The most sensitive parts are the specialized antibodies, which are proteins designed to recognize and bind to the viral antigen. These proteins can denature, or lose their three-dimensional structure, over time, reducing their ability to latch onto the target viral protein.
Degradation is accelerated by exposure to environmental factors such as excessive heat, direct sunlight, or high humidity. The liquid buffer solution is also a vulnerable component, as its chemical composition is precisely calibrated to maintain the optimal pH for the antibody-antigen reaction. If the liquid evaporates, the concentration of the buffering salts changes, disrupting the necessary environment. This chemical breakdown means that even if the virus is present in the sample, the necessary reactions may not be completed, leading to an unreliable result.
False Positives Compared to False Negatives
When a test expires and its components degrade, the overwhelming risk is a false negative result. A false negative occurs when the test fails to detect the virus despite the person being infected. This happens because the detection or capture antibodies become too weak to bind the viral antigen. If the antibodies do not bind, the colored nanoparticles do not accumulate at the T-line, incorrectly indicating a negative result.
Conversely, a false positive—where the test indicates infection when none is present—is highly improbable due to simple chemical expiration. For an expired test to produce a false positive, the degraded antibodies would have to bind non-specifically to other proteins or substances in the sample. Alternatively, the colored nanoparticles would have to spontaneously aggregate at the T-line. The natural process of chemical breakdown over time does not create the conditions necessary for a false positive signal to appear. The loss of antibody function is a failure to react, rather than a misdirected reaction.
Locating the Current Shelf Life
The expiration date printed on the test box often reflects the date determined when the product was first authorized. This initial date was based on preliminary, short-term stability data. However, manufacturers have continued to conduct stability studies, showing that the test components remain functional for a much longer period than initially anticipated. Based on this new data, regulatory bodies like the U.S. Food and Drug Administration (FDA) have granted extensions to the shelf life of numerous authorized at-home tests.
This means a test with an “expired” date printed on the packaging may still be valid for use. To determine the true, current shelf life, check the regulatory agency’s website or the manufacturer’s official product page. Consumers can search for their specific test brand and the lot number printed on the box to find the updated, extended expiration date. This ensures they are not discarding a functional test kit.