The FDA Just Gave This Major Warning About USPS Deliveries

The U.S. Postal Service (USPS) has been working overtime to get more and more mail to people across the country over the past few months. According to the agency, it had to deliver more than 13.2 billion letters, cards, flats, and packages between Thanksgiving and New Year's Eve, greatly exceeding the amount it had to send during the same timeframe in 2020. But if you're waiting for an important package from the USPS, you may want to heed the latest warning from the U.S. Food and Drug Administration (FDA). Read on to find out what you need to know before your next delivery.

RELATED: The USPS Just Issued This Major Warning to Customers.

The USPS might deliver your at-home COVID tests in freezing temperatures.

Under a new initiative from the Biden administration, the USPS is responsible for shipping and delivering free at-home COVID tests to people that have ordered them online. Each household across the U.S. is limited to one order of up to four tests, and some of those packages should be arriving any day now. People have been able to sign up for the program since Jan. 18, and the USPS said the tests would ship within seven to 12 days. With that in mind, residents of certain states might get their package delivered in freezing or below freezing temperatures, according to local news stations across the U.S.

Some officials note that this could be a problem given that many COVID test kits have storage requirements. "The test themselves state right on the packaging that the temperature should be anywhere between 35 to 36 degrees Fahrenheit up to 86. That is kind of in that non-freezing range. Not too cold, not too hot," Shaun Grannis, MD, the vice president of data and analytics at the Regenstrief Institute in Indianapolis, Indiana, told NBC-affiliate WTHR.

The FDA is warning users not to use a test that is still cold.

Your tests being delivered or left outside in freezing temperatures won't automatically make them unusable, according to the FDA. "Since shipping conditions may vary, test developers perform stability testing to ensure that the test performance will remain stable when tests are stored at various temperatures, including shipping during the summer in very hot regions and in the winter in very cold regions," the agency explained in a Jan. 22 update on its website.

But the FDA is warning users that they should not use the test while it is still cold. According to the agency, "test performance may be impacted" if you use it while it is still cold, especially if it has been left outside in freezing temperatures. This means you should not use it outdoors if temperatures are cold or use it right after bringing it inside from the cold.

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If your tests have been delivered in freezing weather, you should wait to open them.

If your USPS package arrives while your area is undergoing freezing temperatures, there are a number of things you can do to "ensure appropriate test performance," according to the FDA. The agency says you should first bring the package inside your home and leave it unopened at room temperature for at least two hours. Once this allotted time has gone by, your package should be good to open and use.

"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," the FDA explains. "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."

But some officials say you might need to toss your frozen tests.

Many households across the U.S. are relying on these free tests from the White House, but taking an inaccurate test won't do you any good. According to Grannis, a test being left outside of its proper storing temperature range for an extended amount of time could result in the liquid inside its kit freezing. The proper environment temperature for an at-home COVID test is 59 to 86 degrees Fahrenheit, per the FDA.

"If they are not kept in that range, what can happen is there are proteins in the liquid that can break down, which is called denaturing, and can become denatured. So we need to be careful with that," Grannis said. "When liquid freezes, it creates ice crystals and those ice crystals can do a lot of damage to the proteins in there, and heat can do the same."

This liquid is usually contained in a pouch or container. If it frozen, it can create a false negative result, according to Grannis. "If the liquid freezes, the advice is not to use it because it's likely defective," he warned.

And while you might still be able to thaw it and use it, Alaska's chief medical officer Anne Zink, MD, advises against using any test that has been frozen more than once. "Avoid using after multiple freeze-thaw cycles; based on manufacturer information so far, we think it is likely the tests will work after one freeze cycle," Zink tweeted on Jan. 20. "If you are ill and the test is negative, consider getting a different test."

RELATED: If You Get This Message From the USPS, Don't Open It.