Those COVID tests the government sent you might not work. Cold weather could be to blame.

At the same time most Americans are facing cold fronts and winter storms, they're also expecting their at-home COVID-19 tests from the government to arrive in the mail.

Most at-home COVID-19 test brands recommend storing the tests above 35 degrees. The liquid reagent inside the cartridge that comes with the at-home tests is susceptible to freezing, and if that happens, the accuracy of the results decreases, Cindy Pins, associate professor of epidemiology at the University of Florida, told USA TODAY.

With the federal government launching a program to send free at-home COVID-19 tests to Americans who sign up through the website COVIDTests.gov, 1 billion tests have been ordered for distribution via the U.S. Postal Service.

But could prolonged cold or freezing temperatures affect the results of the government-sent tests? It depends on how long it's been cold, experts said.

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What does it mean if your at-home tests comes in the mail and it's cold?

Dr. Geoffrey Baird, Chair, chair of laboratory medicine and pathology at the University of Washington's School of Medicine, said the issue with the at-home tests is that if the liquid inside the cartridge is frozen, the results can be skewed.

If the test is outside for a few hours, odds are the test is fine, although not as accurate as it once was. If your test is in your mailbox for a day or more, Baird advises using a PCR test instead. Baird said if your test spent a night in your mailbox in 25-degree weather, it may be best to order another test.

"Just as anything with liquid, if it's chilled or frozen, it changes. That's the same with these at-home tests," Baird said. "At a time where temperatures are freezing in most places, it's safer to choose another test."

Most studies have found a change in the temperature of the at-home test may result in a false-negative test rather than a false-positive, she added. She said if someone is exposed and tests negative with the at-home test, they should receive a PCR test to confirm the results.

"The sensitivity to coronavirus on these tests can be decreased with temperature changes, and it's always tricky to tell by how much," Pins said. "But more often than not, a wrong test shows a false-positive, so then you should get a PCR test."

Antigen tests exposed for extended periods of time to temperatures below 36 degrees or above 86 degrees can deliver inaccurate results, according to a study published by the National Institutes of Health.

“Consequences may include false-negative test results,” the researchers wrote. “Storage and operation of [antigen tests] at recommended conditions is essential for successful usage during the pandemic.”

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What to do if your test comes in the mail cold or frozen

As soon as you get an email confirming your at-home test has arrived, Pins recommended immediately storing it indoors. If the test arrives cold, the room temperature environment will help thaw the liquid inside. But Pins said not to place the test in immediate sun because high temperatures can also affect the results.

"Your best bet is room temperature. Nothing colder or hotter because the test is sensitive to those temperatures," Pins said.

Baird recommends waiting at least four to five hours before using the cold or frozen at-home test. Fortunately, the packaging of the tests was created to be durable through the shipping and arrival process.

The U.S. Postal Service said tests will ship within 7-12 days of ordering and all orders are mailed through First Class Package Service.

Abbott BinaxNOW recommends that its tests remain between 35.6 degrees Fahrenheit and 86 degrees Fahrenheit, adding "test kit reagents must be at room temperature before use," a spokesperson for Abbott told USA TODAY.

However if a test is stored outside the recommended temperature "for a relatively short period of time," it will be fine to use after being placed back in room temperature, the spokesperson said.

If the test lines on your at-home test appear in the incorrect order or color, Baird said that's an indicator it's been contaminated by the weather.

"When you freeze and thaw something, the solutions in it may not actually get back into solution as it once was before. That's where the inaccuracy in these at-home tests can come from," Baird said.

Follow Gabriela Miranda on Twitter: @itsgabbymiranda

This article originally appeared on USA TODAY: At-home COVID tests shipped through cold weather might not be accurate

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