If you stocked up on at-home Covid-19 rapid antigen testing kits during one of the Omicron-variant surges, you could be in for a surprise when you bust them out for a pre-holiday test: an expiration date months in the past. According to the Food and Drug Administration, most at-home Covid-19 tests list an expiration date between four and six months from when they were manufactured.
Does that mean those expired tests no longer work? Here’s why the expiration date printed on the box isn’t as straightforward as you might expect.
When does my at-home, rapid test Covid test expire?
The reason the answer to “when does my at-home Covid test expire?” isn’t as straightforward as, say, when the yogurt in your fridge expires has to do with the tests’ relative novelty. The tests have only been approved for two years, and crucially, the testing they underwent before hitting the market happened in just one year. If the FDA wanted to determine the maximum length of time the test was effective before putting it on the market, it would have had to delay those tests for years.
What it did instead is take a more conservative approach: authorizing a shelf-life of four to six months and then extending the expiration date as real-world data was compiled.
In an email, a spokesperson for the California Department of Public Health told Inverse that “since initial approval, most tests have had specific extensions issued by the FDA because test companies have presented information to the FDA showing the tests work well for a longer period of time.”
How can you tell if your test still works? In January of this year, the FDA released a list of updated expiration dates for different at-home rapid Covid-19 tests.
For example, BinaxNOW rapid Covid-19 tests manufactured by Abbott have been given an updated shelf life of 15 months, as have iHealth rapid tests. FlowFlex rapid tests manufactured by ACON Laboratories have been given a shelf life of 19 months. You can find a full chart of the FDA’s updated shelf-life and expiration dates for different rapid tests here.
California’s Department of Public Health has gone even further, authorizing the use of at-home rapid tests past the FDA’s extended expiration date. Their guidance now says you can use the tests as long as the control line is “both easily visible and the color specified by the specific test instructions” after the 15-minute test-development window. In other words, if you take an expired test and the control line shows up clearly, you can consider that result as valid as an at-home rapid test that isn’t expired.
How to prolong the shelf-life of your at-home rapid Covid-19 test
A longer than initially anticipated shelf life of at-home Covid tests doesn’t mean your tests will necessarily last that long. Like medication, proper storage of Covid tests will prolong their shelf life.
If the test is exposed to very hot or cold temperatures, it will degrade faster than if it’s kept at more stable temperatures. According to the FDA, “test performance may be impacted if the test is used while it is still cold, such as being used outdoors in freezing temperatures or being used immediately after being brought inside from freezing temperatures, or in a hotter than expected environment, such as outside in the summer.”
To perform as expected, manufacturers assume the test is being performed in an environment that is roughly between 59 and 86 degrees Fahrenheit. If the test is delivered to you when it’s very hot or cold outside, the FDA advises bringing the test inside and leaving it at room temperature for at least two hours before using it.
If you’re not sure if the test is still good, the FDA suggests taking the test. “As long as the test line(s) appear as described in the instructions, you can be confident that the test is performing as it should. If the line(s) do not appear in the correct location(s) and within the correct time as shown in the test instructions when you perform the test, then the results may not be accurate, and a new test is needed to get an accurate result,” the FDA explains.
How rapid tests work and what goes bad when an at-home Covid-19 test degrades
Nate Hafer is the director of operations for the University of Massachusetts Center for Clinical and Translational Science and an assistant professor in molecular medicine at UMass Chan Medical School. He tells Inverse that the extended expiration dates make sense to him, especially in light of how the tests work.
After you swab your nostrils, you put the swab in a solution that breaks apart the sample so different proteins in the sample can be detected. In the case of rapid antigen tests, the protein they’re looking for is called the nucleocapsid protein, which is “one of the most abundant proteins in the SARS-CoV-2 virus,” Hafer says.
Once the solution has broken apart the genetic material, you put a few drops of the solution on the test strip. The strip has been painted with antibodies that, if they come into contact with the nucleocapsid protein, will change color. As the solution moves down the paper, the control line will appear regardless of whether it detects the nucleocapsid protein or not.
The various substances used in the test are fairly stable, Hafer says, which is why the tests last past the expiration date if stored properly. If they are exposed to prolonged heat or cold, Hafer says both the solution and the antibodies on the test paper could degrade.
“I would guess that the antibodies on the strip of paper are one of the more sensitive things that could be damaged by extreme heat or cold, but the solution probably has some chemicals in it that could also be influenced by extreme temperatures,” he says.
So if you come across an expired at-home test, don’t immediately throw it away. Hang on to it and the next time you need to test, make sure the control line shows up as it should. If it does, go ahead and treat it like a non-expired test. If that line looks funky, head to the store for a new one.