COVID testing FAQ: Here's everything you need to know : Shots ...

As Susan Butler-Wu, associate professor of clinical pathology at the Keck School of Medicine of University of Southern California, puts it: "It's a test for [determining whether you have] a lot of virus."So you might test negative on a home test even if you are infected — at the beginning or the end of your illness, for example, when you don't have a lot of virus.The more urgent question, says Butler-Wu, is: "Which test can you get?"If you have symptoms and likely have been exposed to the virus by traveling or socializing, a positive antigen test is probably enough evidence that you have the virus, says Dr. Abraar Karan, an infectious disease physician at Stanford University.As for PCR tests, availability depends on the demand in your community. And it can take several days to get results from a PCR test.While you're waiting for test results, if you have symptoms you should act as though the test is positive and quarantine.If you've been exposed but don't have symptoms, the CDC says you can go out while wearing a well-fitting, protective mask if you're vaccinated and boosted.Those who aren't vaccinated should quarantine after a known exposure until test results are in.(See this story for guidelines about quarantining or self-isolating after an exposure.)The answer depends on whether you can get tests — and what you're using them for. If you test positive on a home test, you should notify the health department so it can keep track of how many cases your community has.False positives are rare on PCR tests and usually happen because of contaminated samples, research has found.A false positive on an antigen test is possible but fairly unlikely if the test is taken correctly, says Butler-Wu, especially if you develop symptoms and you know you've been exposed to someone with COVID-19. And a lot of people are being exposed at this current time of great spread to the omicron and delta variants.If "there's a bunch of COVID and I'm symptomatic, it's probably a true positive," Butler-Wu says.If you think you have a false positive from a home test, you can get a confirmatory PCR test if you can find one."If your PCR is negative, then it's possible that your rapid was a false positive," Karan says. And some workplaces and schools require a negative PCR test to return after travel or a non-COVID illness.Rapid tests may not be as accurate for omicron, the U.S. Food and Drug Administration said in late December — but they haven't released data yet on why they are less accurate and to what degree.The FDA has also warned that three types of PCR tests may not detect omicron.Because of these issues, if you're testing at home after symptoms or an exposure to someone with COVID-19, the use of two tests spaced a few days apart is critical.No. You should complete at least five days of isolation after you test positive.According to the CDC, you may end isolation five days after testing positive, as long as your symptoms are "resolving," including no fever for 24 hours, and you wear a mask "at all times when around others" for five more days.The new guidance prompted criticism from public health experts. "I'm just looking at how often people who have COVID still shed infectious virus at five days, and it's quite a chunk."On Tuesday, the CDC changed those recommendations to include taking a test on Day 5, if possible, but it stopped short of requiring a test.But if you do test at that point and test positive, continue isolating until you test negative."If you have COVID, and your antigen [test] is still positive at five days, you're almost certainly still infectious," Butler-Wu says.And viral load isn't the only factor for transmitting the virus.