What are the chances of a false positive rapid test

Toronto -

As provinces rely more heavily on rapid antigen tests as part of their strategy to curb the spread of COVID-19, there have been concerns over the possibility of false positive results. Canadian researchers have produced new data shedding light on the likelihood of such events.

Researchers from the University of Toronto's Rotman School of Management published their peer-reviewed findings in the journal JAMA earlier this month. They looked at the results of more than 900,000 rapid antigen tests conducted over 537 workplaces in Canada between January and October 2021.

During this period, Canada had two significant waves of COVID-19 driven by the Delta variant. A total of 1,322 positive results were logged with rapid tests. Of these cases, 1,103 also had data from a PCR test to compare against.

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In total, 462 rapid test results, or 0.05 per cent of the 900,000 results, resulted in false positives. This represents 42 per cent of the positive test results in the study.

Tests being administered too late in the infectious state or in an incorrect manner were some of the reasons that could explain these false positives, the researchers say.

Around 60 per cent of these false positives could also be traced back to issues stemming from one manufacturer. There were 278 false positive results from two workplaces that were all drawn from a single bad batch.

"The overall rate of false-positive results among the total rapid antigen test screens for SARS-CoV-2 was very low, consistent with other, smaller studies," the researchers wrote. "These results inform the discussion of whether rapid antigen tests will result in too many false-positives that could overwhelm PCR testing capacity in other settings.

The authors also say their findings illustrate the importance of "a comprehensive data system" to quickly identify bad batches of rapid tests.

"With the ability to identify batch issues within 24 hours, workers could return to work, problematic test batches could be discarded, and the public health authorities and manufacturer could be informed," they wrote.

At-home COVID-19 testing has been around since early last year, but as the latest Omicron subvariants like BA.4 and BA.5 continue to emerge and spread throughout 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.

Fortunately, at this point in the pandemic, you can now order free rapid tests to be sent to your door. Every home in the U.S. is eligible to order a 3rd round of free at-home tests. Order yours here.

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.

False negatives are more likely than false positives.

The false positive rate on rapid antigen testing is very low. One study estimated that 0.05% of positive tests are false positives, says Richard Watkins M.D., an infectious disease physician and professor of internal medicine at the Northeast Ohio Medical University in Rootstown.

It’s important to always check the expiration date on your at-home rapid tests, because yes, they do expire. The FDA does not recommend using at-home COVID-19 diagnostic tests beyond their authorized expiration dates. COVID-19 tests and the parts they are made of may degrade, or break down, over time. Because of this, expired test kits could give inaccurate or invalid test results.

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.

The PCR test can rarely be a false positive, says Dr. Watkins, but “in an asymptomatic person without known close contact with an infectious individual, especially in a low prevalence setting, the finding of a positive COVID-19 PCR test should raise the possibility that the result might be a false positive.”

“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.

False positive results on home Covid antigen tests are rare, especially when it is someone who is symptomatic, says Amesh Adalja, M.D., a senior scholar at the Johns Hopkins Center for Health Security. So if you are taking a test because you are already feeling under the weather, it’s safe to say that your positive result is indeed a “true” positive.

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.

Issues related to timing of testing (i.e. too early or too late in the infectious stage) can also cause false positive results, says Dr. Watkins. According to the latest from the CDC, if you have COVID-19 symptoms, test yourself immediately. If you were exposed to someone with COVID-19, test yourself at least 5 days after your exposure. If you test negative for COVID-19, consider testing again 1 to 2 days after your first test.

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.”

You may be eligible to receive antiviral therapy, such as Paxlovid, says Dr. Watkins. “If you have also had contact with someone who has COVID-19 and you test positive, let your health care provider know,” he adds.

“We’re seeing lower positive test rates among people who have been vaccinated and boosted.”

“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 various Omicron subvariants of COVID-19 is to get your initial vaccinations and boosters 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 can continue to keep transmission levels and hospitalizations low and soon, possibly see the transition from pandemic to endemic in our sights.

Jake Smith, an editorial fellow at Prevention, recently graduated from Syracuse University with a degree in magazine journalism and just started going to the gym. Let's be honest—he's probably scrolling through Twitter right now.

Madeleine, Prevention’s assistant editor, has a history with health writing from her experience as an editorial assistant at WebMD, and from her personal research at university. She graduated from the University of Michigan with a degree in biopsychology, cognition, and neuroscience—and she helps strategize for success across Prevention’s social media platforms. 

Can a rapid test give you a false positive?

Can Covid 19 antigen tests be false positive?