CHAPEL HILL, N.C. (WNCN) – With cases rising, many people are turning to rapid COVID-19 tests before holiday gatherings or when they come down with symptoms. We wanted to know how accurate those tests are.

You’ve probably heard of someone testing negative one day and positive the next, or even negative in the morning and positive in the afternoon. While rapid tests aren’t perfect, health experts said they’re a valuable tool.

Dr. Dirk Dittmer, who runs UNC’s coronavirus sequencing lab, compares a rapid test for COVID-19 to the gas light in your car.

“They have a threshold,” he explained. “It really only turns on if you have above the threshold amount of virus. Or, in your car, below the threshold amount of gasoline.”

That means if you have a small amount of virus in your body, it may not show up on a rapid test at first. If you know you’ve been exposed to COVID-19 or if you have symptoms, Dittmer said you may need to repeat the test a few times.

“If I have enough tests, I would test every day,” he said. “The virus does take a while from that initial infection event, which could be just one single particle, until it replicates enough in your nose give a positive test.”

“What we know is if you test positive, you are infectious. What we don’t know as much is the day before you test positive, when you test negative, you probably are infectious as well,” he added.

While testing before an event or gathering may not catch every single person who may become infectious, Dittmer said it is still a good idea to test that day.

“Any time before you go to the party is fine,” he said. “I would feel safe, to be honest. There will be people who test negative at 8 in the morning and they start shedding the virus at midnight, and no test can get you that kind of accuracy.”

Dittmer said you can improve the accuracy by carefully following the directions on the test and making sure you get a good sample. If you do test positive, he said it’s important to call your doctor right away to find out about treatment options.