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At-home tests for COVID-19 are quick, easy, and convenient. Plus, they produce results in 15 minutes, so you’ll know almost immediately if you need to take action. But if you have an expired COVID test sitting in the hallway closet, you might be wondering if it’s still okay to use. While a COVID test won’t magically go bad the day it expires, most experts warn against using an expired at-home test too long after the date on the box.

According to the Centers for Disease Control and Prevention (CDC), a positive self-test detects a current infection. If the test is negative, that means it did not detect the virus, but it does not rule out an infection. That’s why many manufacturers recommend repeat testing 24 to 48 hours later.

The exact time from when the product is manufactured to when it expires varies by manufacturer. This expiration date is usually based on the stability of the reagent and antibodies used for the tests. If you’re not sure that you should use an expired COVID test, here’s what you should know.

When do COVID tests expire?

All at-home rapid antigen COVID tests have expiration dates printed on the box. You may need to do some hunting around to find it, but in general, the first place to look is on the back of the package. The other location to scope out is the front. For example, the BinaxNOW test has a sticker with the expiration date front and center printed next to an hourglass icon.

With that said, some manufacturers have recently extended these dates by up to three months. To find out if you have an expired test that may still be good to use, contact the manufacturer or visit their website for info. You will likely need the lot number to determine if the date is extended. You can also check the FDA’s list of extended expiration dates here. If you cannot find any information about an extension to the expiration date, a good rule of thumb is to stick with the date on the package.

Why do COVID tests expire?

The reason why COVID tests have an expiration date is not explicitly stated in manufacturer package inserts, but Erika Gray, Pharm.D., the chief medical officer and co-founder of Toolbox Genomics, says this expiration date can reflect how comfortable each manufacturer is in guaranteeing their product will work as it should when factors such as temperature changes during transport and the way individuals store the tests at home are considered.

Robert Stone, MD, the senior medical director of ambulatory services at Central Ohio Primary Care, says at-home tests expire since the reagents (chemicals) that the test uses become less active over time, just like food gradually becomes less flavorful or spoils.

In addition, Mary Valvano, MD, the co-founder of BetterNowMD, says test kits can also expire when exposed to air and water molecules in the air or temperature changes. “Exposure to these elements affects the various components of the kit,” she says, “most specifically the reagent, but also the various test papers and things that the kits are made out of.”

Can you use an expired COVID test?

Although it’s not dangerous to your health, per say, to use an expired COVID test, the results would not be considered reliable. Using an expired COVID test increases the risk of a false positive or false negative test result. “Using the test could cause you to falsely think you don’t have COVID, for example,” Dr. Stone says, “so you would not know you had it and could potentially expose others.”

The package insert that comes with an at-home COVID test states not to use an expired test as it may give inaccurate results. “This is an evolving situation, however, since at-home COVID tests are a relatively new technology and research on their stability over time is continuously being done,” Dr. Gray says. Because of that, there are situations where the expiration date is extended, and tests are considered accurate.

According to the FDA, the CareStart COVID-19 Home Antigen Tests distributed by the federal government are safe to use three months after the expiration date on the box. They also extended the expiration date for other home COVID tests based on stability tests performed by manufacturers. One example of this extension, says Dr. Gray, is for Abbott’s BinaxNOW home COVID tests, which changed the date from 12 months post-production to 15 months post-production.

With those extensions in mind, if you decide to use an expired COVID test, “it’s important to confirm the test result with either a non-expired test or a PCR test, especially if you’re going to be around at-risk individuals or going to school or work,” Dr. Valvano says.

How should you store a COVID test kit?

Storing a COVID test kit correctly may help preserve the test and ensure the results are accurate when taken before the expiration date. When you get a test kit, the first order of business is to read the instructions that come with it and look specifically for storage instructions. Some manufacturers print everything on the box, while others include an insert in the package.

In general, most at-home antigen tests are stable in a wide temperature range, as long as it is not too cold or too hot. For example, the instructions for the BinaxNOW test advise storing the kit at 35.6 to 86 degrees Fahrenheit (F). When you get a test—whether it’s delivered by mail or you purchase at a store, bring it inside as soon as possible and store it in a dry, room-temperature location.

In addition, Dr. Valvano says most manufacturers advise letting all test kit components come to room temperature, if necessary, before using. And finally, when it comes time to take the test, try to complete all steps indoors where the temperature is stable.

Ultimately, if you’re unsure about using an expired COVID test, call your healthcare provider or the manufacturer directly. They can advise you on whether or not you should use the test or purchase a new one.