What Is a False Positive COVID-19 Test? Everything to Know About At-Home Testing

·6 min read
What Is a False Positive COVID-19 Test? Everything to Know About At-Home Testing

At-home COVID-19 testing has been around since early last year, but as the Omicron variant surges through the United States, it’s never felt more convenient—or more important. When you can get your hands on one, a rapid antigen test allows you to skip hours-long lines for professionally administered PCR tests, getting results in just minutes.

But how accurate are antigen tests? False positive COVID-19 tests—when your result is positive, but you aren’t actually infected with the SARS-CoV-2 virus—are a real, if unlikely, possibility, especially if you don’t perform your at-home test correctly.

Don’t dispose of your stash of at-home COVID-19 tests, though: “[False positives] are not very common at all,” explains Gigi Gronvall, Ph.D., a senior scholar at the Johns Hopkins Center for Health Security, where she has led efforts to track the development of COVID-19 testing. “It happens, but it is extremely rare.”

Rapid tests are even more trustworthy now, with so many people infected with the novel coronavirus, says Geoffrey Baird, M.D., Ph.D., professor and chair of the Department of Laboratory Medicine and Pathology at the University of Washington School of Medicine. Before you can assume your positive COVID-19 test is a fluke, you need to understand why false positives exist in the first place.

What is a false positive COVID-19 test?

In the most basic sense, there are four possible outcomes for a COVID-19 test, whether it’s molecular PCR or rapid antigen: true positive, true negative, false positive, and false negative. “True” and “false” refer to the accuracy of the test, while “positive” and “negative” refer to the outcome you receive, Dr. Baird explains. Therefore, “false positive” means that you have been delivered a positive result, but are not actually infected with the SARS-CoV-2 virus.

How common are false positive COVID-19 tests?

First, a crash course in virus testing vocabulary: “Sensitivity” measures a test’s ability to accurately identify people who have COVID-19, Dr. Baird says. “Specificity,” meanwhile, refers to a test’s ability to correctly identify people who do not have the virus. Specificity will generally be higher than sensitivity, especially when people have COVID-19 symptoms—in other words, false-negative COVID-19 tests are more likely than false positives.

All rapid tests currently authorized for home use by the Food and Drug Administration (FDA) have high sensitivity and specificity, meaning they have a high accuracy rate. For example, Ellume reports 100% specificity in symptomatic people and 96% specificity in asymptomatic individuals. Flowflex demonstrated 100% specificity during FDA testing. And BinaxNOW antigen tests had up to 99.7% specificity during real-world testing.

The “gold standard” of COVID-19 testing is PCR, also known as molecular testing, Gronvall says. However, antigen tests can be just as sensitive as PCR ones when you’re experiencing symptoms, she explains. Although some kits allow you to perform PCR tests at home, often with the option of sending samples to a lab, you’re more likely to have access to rapid antigen tests given the current barriers to testing.

“Despite the high specificity of antigen tests, false positive results will occur,” the Centers for Disease Control and Prevention (CDC) writes. “In general, for all diagnostic tests, the lower the prevalence of infection in the community, the higher the proportion of false positive test results.” Put another way, false positive results will always occur—there’s no way around it, Dr. Baird explains. But again, they are not common.

“The specificity isn’t the problem right now,” he continues. “The problem with [at-home tests] is actually the other side, the false negatives, the fact that they’re not very sensitive.” Antigen tests are most accurate when you have symptoms, Dr. Baird says, since that usually correlates to having “a lot of virus” in your body—it’s easier for the tests to detect.

How can you avoid false positives from rapid COVID-19 tests?

Antigen COVID-19 tests require you to swab your nostrils to collect a sample—but the goal isn’t to pick up mucus. “A lot of folks think that what they’re trying to do is dig as deep as they can,” Dr. Baird explains. “That can actually cause some false positives.” Snot, hair, blood, and other extras might interfere with your test’s ability to identify SARS-CoV-2 antigens.

“We want the swab to scrape off the superficial layer of cells [in the nose],” he continues. “That’s where the virus is associated with. That’s what we want.” If you’re planning on testing yourself, it’s not a bad idea to wipe or blow your nose to make sure you’re collecting cells rather than snot, he says. (Just be mindful not to blow your nose if you’re feeling sick and other people are in the same room.)

Perform “gentle, but firm circles” in each of your nostrils, Dr. Baird recommends. Another important step is to follow the respective test’s instructions as closely as possible: Use the correct amount of drops, check the test when it tells you to, and resist the urge to skip any steps. (Don’t swab your throat, either, at least if you only have one test on hand.) At-home tests aren’t ideal for people with disabilities and those with impaired vision, he says, so it might be helpful to have someone else help you—if that’s possible.

And one of the simplest ways to confirm your result is just to perform a second test, Gronvall notes. Given a nationwide shortage of test kits, that can be easier said than done, but if you have the resources and need to be sure you aren’t infected, it’s worth the extra effort.

What should you do if you test positive on an at-home test?

If you test positive at home, don’t assume it’s a false positive, especially if you’re experiencing the symptoms of COVID-19. “You should stay home or isolate for 10 days and wear a mask if others could have contact with you,” the CDC recommends. “Also, tell a healthcare provider about your positive test result and stay in contact with them. If your illness becomes severe, seek medical attention.”

“Given that so many people have COVID-19, if you have symptoms and you test positive by an antigen test, don’t bother getting a PCR test right now,” Dr. Baird says. “The chances of it being one of those false positives are probably pretty low.” If you’re asymptomatic or mildly symptomatic and you have the time, though, he notes that a second test might be worthwhile.

“If you feel symptoms but test negative, you should hold off on celebrating and retest in a couple of days,” Gronvall recommends, “particularly if you have been exposed to a positive case (which is almost everyone these days).” When you feel crummy and leave your home, you could still potentially spread SARS-CoV-2, a cold, or the flu to the people in your vicinity.

Above all else, the best way to protect yourself from the Omicron variant of COVID-19 is to get your initial vaccinations and booster if you’re eligible, he explains: “We’re seeing lower positive test rates among people who have been vaccinated and boosted.” Through prevention measures like vaccinations, testing, and masking, we could soon start to get a handle on the Omicron wave.

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