(NEW YORK) — With COVID-19 cases rising across the country, you may be inclined to pull out one of those leftover at-home rapid tests received months ago from the Biden administration.

Even if the box says the tests have passed their expiration date, experts told ABC News you may want to think twice before throwing them away.

They say the expiration date may have been extended by the federal government and they could come in handy if you need them in a pinch.

How is the expiration date determined for an at-home COVID-19 test?

Expiration dates are determined by the manufacturer, typically with a shelf life of about four to six months from the manufacturing date, which is then authorized by the U.S. Food and Drug Administration.

Dr. Matthew Binnicker, director of clinical virology at the Mayo Clinic, told ABC News that earlier in the pandemic, expiration dates were conservatively set due to unknowns about how long they would be effective for.

“Now that we have been in the pandemic for over three years, the manufacturers have had a better opportunity to determine the true expiration dates of those kits,” he said.

“And the FDA has actually published extended expiration dates based on additional data from manufacturers, so the expiration date that’s on a test or on a box of antigen tests may not actually be the true expiration date. People should go on to the FDA website,” Binnicker said.

To check for an extended expiration date, visit the FDA webpage on at-home tests, which indicates the brands that have an extended date, and a PDF for each test with new expiration dates.

You can find the lot number on the package, near the expiration date, and then check the PDF to see if it’s among those dates.

What does it mean if my test is expired?

If you confirm an at-home expired and has not been extended, it is best to find a new COVID test that still has an existing shelf life.

However, it doesn’t mean it can’t be used.

“The further you get from the expiration date, the less likely it is to perform exactly the way that you expected it to perform so you might want to be slightly more suspect,” Dr. Emily Landon, an associate professor of medicine at UChicago Medicine, told ABC News. “But honestly, I don’t think there’s anything wrong with using a test that’s a little bit past its expiration date.”

She said if the test performs strangely — such as the liquids used in the test looking cloudy or a first line not appearing — those tests should be thrown away. But it could be helpful to use in a pinch.

“But if you have old tests, I certainly think that using old tests is better than using no tests,” she said.

Binnicker added, “if you do [used an expired test] and you get a positive result, it’s typically going to be a reliable result. If it’s negative, it might not be reliable.”

Do old tests work against the new variants?

The FDA and experts say that older at-home tests work for detecting COVID infections caused by new variants, most of which are omicron offshoots.

What makes most of the newer variants different is mutations in the spike protein, which is what the virus uses to enter and infect cells, Binnicker explained.

But “the antigen tests typically look for a different protein on the virus called the nucleocapsid protein,” he said. “So the data that are available suggests that the antigen test should continue to be effective in detecting the more recent circulating strains.”

When should I test for COVID-19?

If you are experiencing symptoms, experts recommend taking a COVID test right away.

For those recently exposed to someone who tested positive, take a test five full days after you last interacted with the person, according to Centers for Disease Control and Prevention guidelines.

If you plan to come into contact with those who may be at high-risk of severe disease, such as elderly individuals or those with compromised immune systems, you may opt to take a test, the experts say.

“The number one thing to remember is that if you have any symptoms or respiratory virus, you really need to stay away from others or wear a mask when you’re around them,” Landon said.

If positive, try to stay home, isolate yourself from others, and follow-up with your health care provider.

Binnicker said a person with a positive rapid test and symptoms does not need a confirmatory PCR test but that it could help for a person with a positive test who is asymptomatic.

I got a negative test result on an at-home COVID-19 antigen test. Do I need to take another test?

Despite a negative test, you may want to test again in the coming days, as some tests may not detect the virus early in an infection.

Try testing 48 hours after your first negative test if you have symptoms. Those without symptoms should test 48 hours after the first negative test and then 48 hours after the second negative test.

“This is where I think most people make the mistake is they feel that the tests are less reliable because it takes until their second or third day of symptoms for them to have a positive test,” Landon said. “But that is expected…The home tests are not meant to detect the very first signs of COVID. They’re supposed to be able to tell if your respiratory illness is COVID or isn’t COVID.”

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