Can you still use an at-home COVID-19 test past its expiration date? It depends.

Once again, COVID-19 is on the rise, and this time, the highly contagious BA.5 Omicron subvariant is driving new cases and reinfections. But if you stocked up on at-home rapid antigen tests during previous surges, you may have noticed the fine print on the packaging shows your tests are past due.

Don’t throw them out just yet.

The Food and Drug Administration has advised against using at-home COVID-19 tests beyond their expiration dates, warning that expired test kits could provide unreliable or invalid results. But the expiration dates — printed on the outside of the boxes — are not always accurate.

That’s because the FDA has recently extended the shelf-lives of several popular test kit brands upon reviewing additional data from their manufacturers.

“Like everything with COVID-19, there are decisions that are made in an absence of data, and when companies in our country make a product like a test, they have no way of knowing, when it’s new technology, how long it’s going to be valid for,” said Dr. Shira Doron, an infectious disease physician and epidemiologist at Tufts Medical Center.

Test manufacturers determine the shelf life of their products with a process known as “stability testing” which, according to the FDA, requires evaluating the tests’ performance in real time. The expiration date is the date through which the test should perform as accurately as when it was first manufactured.

But the testing materials can dry out or degrade over time, compromising the validity of the test results, Doron said. So if a shelf-life extension for your at-home test has not been authorized by the FDA, don’t use it.

“The risk of using an unapproved and expired test is accuracy in both directions — false negatives and false positives,” Doron said. “Inaccurate test results are harmful. You could get treatment you don’t need because you think you’re positive, or you could miss out on treatment you need if you think you’re negative.”

Doron said consumers should feel confident using tests with approved shelf-life extensions past their printed expiration dates, but only if those tests have been properly stored. Extreme temperatures, in addition to moisture and humidity, can impact a test’s ability to accurately screen for COVID-19.

“You do want to make sure that test has not been exposed to extreme of weather, heat or cold, that includes potentially if it was in the baggage area of an airplane where it gets really, really cold,” she said. “That might not be one I’d want to use.”

Boston health officials have urged residents to take additional precautions as COVID-19 cases and hospitalizations increase citywide. The Boston Public Health Commission has recommended mask-wearing in crowded indoor spaces, and testing before and after large gatherings.

If you want to know if the shelf-life of your test kit has been updated, visit the FDA’s website, where you’ll find a regularly updated list of all the at-home rapid tests that have been authorized by the agency, along with their new expiration dates.

The FDA has approved expiration extensions for several brands of rapid test kits, according to the agency’s website, including:

Deanna Pan can be reached at [email protected]. Follow her on Twitter @DDpan.