How often are there false positives for covid? The experts answer

We explain how reliable tests are when done correctly

Home tests for covid-19 have become very popular since last year, but since the omicron variant is increasingly contagious, it is convenient (and important) to know how to do these tests well . The rapid antigen test can give you results in a few minutes and not have to wait for hours queuing for a PCR with professional staff.

But how accurate are these tests? A false positive is when the test result is positive but you are not actually infected with the new coronavirus. It happens sometimes, but it's unlikely, especially if you do a good test at home.

First of all, don't throw away your stockpile of home tests thinking "false positives aren't very common," explains Dr. Gigi Gronvall , a specialist at the Johns Hopkins Center for Health, where she has led research to track the development of covid-19 tests. "It does happen, but it's extremely unusual."

In fact, rapid tests are even more reliable now that so many people have been infected with the new coronavirus , argues Dr. Geoffrey Baird , chair of the Department of Laboratory Medicine and Pathology at the University of Washington School of Medicine. He reasons that before assuming your positive on a covid-19 test is false, you need to first understand why there are false positives .

What is a false positive in a covid-19 test?

To summarize, there are four possible outcomes when doing a COVID-19 test , whether it's a PCR or an antigen test: a true positive, a true negative, a false positive, and a false negative. "True" and "false" do not refer to the accuracy of the test but to the result, explains Dr. Baird. On the other hand, "false positive" means that with a possible positive result you really are not infected with the SARS-CoV-2 virus.

How often are there false positives in covid-19 tests?

Let's start with a short course on test jargon:

• "Sensitivity". Measures the ability of a test to identify those who have covid-19.
• "Specificity". It refers to the ability of the test to correctly identify those who do not have the coronavirus.

The second will be more frequent than the first, especially when people have developed symptoms. In other words, there are more chances of false negatives than false positives.

All antigen tests currently authorized by the authorities have a high sensitivity and specificity . For example, Ellume reports 100% specificity on people with symptoms and 96% specificity on asymptomatic people with his tests. For its part, Flowflex has proven 100% specificity in trials with medical authorities. And BinaxNOW antigen tests had 99.7% specificity in a test with individuals.

We know that the gold standard is PCR, also known as a molecular test, although antigen tests can be as sensitive as PCR when you have symptoms , according to Dr. Gronvall. Some kits allow you to carry out PCR tests at home, but since you have to send the samples to the laboratory, it is more likely that you will decide on rapid antigen tests given the current conditions.

"Despite the high specificity of antigen tests, false-positive results sometimes appear," acknowledge the Centers for Disease Control and Prevention ( CDC ) in the United States. "In general, for all diagnostic tests, the lower the frequency of infection in the community, the more likely it is that false-positive results will occur." In other words, false positives are always going to happen, explains Dr. Baird. But again, they are known to be uncommon.

"Specificity is not the problem now. The problem with home tests is actually the other side (false negatives) , when they are not very sensitive," he says. Antigen tests are more accurate when you have symptoms , says the doctor, because the more often you have "a good amount of the virus" in your body, so it's easier to detect.

How can I avoid a false positive in a rapid test?

COVID-19 antigen tests require you to swab your nostrils to collect a sample, but the goal is not to collect mucus . " A lot of people think that the thing to do is dig as deep as possible ," explains Dr. Baird. " That may be the real cause of some false positives ." Mucus, hair, blood, and other additional elements can interfere with the test's ability to identify SARS-CoV-2 antigens.

" We want the swab to scrape off the surface layer of cells [from the nose] ," he continues. "That's where the virus can be found. That's what we want." If you're thinking about getting tested, it's not a bad idea to wipe or blow your nose to make sure you're picking up cells instead of mucus , she says. (Remember not to blow your nose if you have symptoms and other people are in the same room.) Make "gentle but firm circles" in each of your nostrils, Dr. Baird recommends.

Another important step is to follow the instructions of the corresponding test to the letter: Use the correct number of drops, check the indicated test and resist the temptation to skip any step . (There's also no need to clear your throat, at least if there's only one test at hand.) Home tests are not ideal for people with disabilities or vision problems , warns the specialist, so it can be useful to have another person help , if possible.

And one of the easiest ways to confirm the result is to do a second test , says Dr. Gronvall. Given the shortage of test kits at times, this may be easier said than done, but if you have the resources and need to be sure you're not infected, it's worth the extra effort.

What should I do if I test positive in a home test?

If you test positive on a home test, you don't have to assume it's a false positive, especially if you're experiencing symptoms of covid-19. "You should stay at home and isolate yourself for ten days, wear a mask if there are other people who could be close contacts. You should also talk to your doctor or the authorities to communicate your positive and keep in contact with them . If the disease worsens, then seek medical attention", recommend the health authorities .

" Since there are so many people with COVID-19 now, if you have symptoms and you test positive for antigens, don't bother getting a PCR test right now," says Dr. Baird. " The chances of it being one of those false positives are probably pretty low ." However, if you are asymptomatic or mildly symptomatic and you have time, it might be worth doing a second test.

" If you feel symptoms but test negative, you should suck it up before celebrating and retest a couple of days later ," Gronvall recommends, "especially if you've been exposed to a positive case (which is pretty much everyone today). in day)". When you feel bad and leave the house, it is possible that you spread SARS-CoV-2, a cold or the flu to people who are close to you .

“We are observing lower positive rates among people who have been vaccinated and received the booster”

Finally, experts remind you that the best way to protect yourself from the omicron variant of COVID-19 is to get vaccinated and receive a booster dose if you are eligible. "We are observing lower positive rates among people who have been vaccinated and received the booster," says the doctor. With preventive measures such as vaccination, testing and face masks, the omicron surge could soon be brought under control.