COVID-19 rapid antigen tests became a common tool for individuals to check for the virus at home. These tests led to concerns about their reliability once they approached or passed their printed expiration dates.
How Rapid Antigen Tests Function
Rapid antigen tests detect specific viral proteins, antigens, in a sample. A collected nasal or throat swab is applied to a sample pad. The sample then moves along a membrane through capillary action.
As the sample travels, it encounters a conjugate pad containing detection antibodies, marked with colored particles. If viral antigens are present, they bind to these labeled antibodies, forming a complex. This complex migrates along the membrane until it reaches a specific test line.
The test line contains immobilized capture antibodies that bind to the antigen-antibody complex, forming a visible line, indicating a positive result. A separate control line also appears, confirming the test functioned correctly.
Why Expired Tests Lose Accuracy
Expired rapid antigen tests may yield unreliable results, most notably false negatives. This diminished accuracy stems from the degradation of key reagents. The antibodies on the test strip and in the conjugate pad can break down over time.
When these antibodies degrade, their ability to bind to SARS-CoV-2 viral antigens is compromised. This reduced binding efficiency means that even if the virus is present, the test may fail to produce a visible signal at the test line. Consequently, the test might show a negative result, even when the individual is infected.
Environmental factors, like extreme temperatures or high humidity, can accelerate this degradation process, further impacting test sensitivity. The liquid solution included in some test kits can also evaporate over time, leading to insufficient volume for the test to run properly. While the main concern is false negatives, false positives are generally less likely with expired tests because degraded components typically result in a lack of signal rather than a misleading one.
Initially, the U.S. Food and Drug Administration (FDA) authorized rapid antigen tests with conservative expiration dates, often four to six months, due to limited stability data. However, as manufacturers conducted more long-term stability studies, the FDA extended the shelf lives for many tests, sometimes for up to two years, based on data confirming their continued accuracy. Therefore, the date printed on a test box might not always reflect its current, extended expiration date.
Guidance for Expired Test Kits
If you possess COVID-19 rapid antigen tests, check the expiration date on the box before use. This date may not be the definitive expiration, as many rapid tests have received extended expiration dates from the FDA based on additional stability data.
To determine if your test’s expiration date has been extended, consult the FDA’s website. It provides a list of authorized at-home tests and their updated shelf lives. You will need the test’s manufacturer and lot number, usually found near the original expiration date, to verify any extension.
If your test is expired and not listed with an extended shelf life, it is not recommended for use due to the potential for inaccurate results. If you are experiencing symptoms or suspect exposure to COVID-19, obtaining a new, unexpired test kit is advisable.
For situations requiring a highly accurate diagnosis, such as when symptoms persist despite negative rapid test results, a laboratory-based PCR test may be appropriate. Regardless of test results, adhere to public health guidelines, including isolation or masking, if symptoms of respiratory illness are present or exposure is suspected.