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How accurate are rapid antigenic tests?
Two types of rapid tests are used to detect active COVID-19 infection: rapid antigen tests which detect viral proteins using a strip of paper and rapid molecular tests – including PCR – which detect the virus. viral genetic material using a medical device.
It is important to remember that rapid antigenic tests serve a different purpose than PCR tests, which are considered the gold standard even though they are not 100% accurate. Rapid tests are designed to identify cases with a viral load high enough in the nasal passages to be transmissible – not to diagnose all cases of COVID-19. The Abbott BinaxNOW Rapid Antigen Test can only detect 85% of positive cases detected by PCR tests. But the key is that published studies have found that they detect more than 93% of cases that are at risk of transmission, which is what matters most to bringing the pandemic under control. Ellume correctly identifies 95% of all positive cases, and Quidel QuickVue accurately identifies 85%. All three tests correctly identify up to 97% of all negative cases, regardless of symptoms.
How to use rapid tests?
Rapid antigenic tests can be used in three ways to slow transmission. First, people can take a rapid test when there is suspected or known exposure to COVID-19. Second, rapid tests can provide an additional precaution before any activity with a higher risk of transmission, such as gatherings or travel. Third, it’s also possible to test regularly – every week, for example, whether enough tests are available – to detect cases that might otherwise go undetected.
It is important to have a plan of action based on the results of the test. If you test positive, immediately take precautions to slow transmission, such as self-isolation, notifying nearby contacts of the test result and reporting the case to health authorities. Less than 3% of positive results are false positives, but a second rapid test the next day or a PCR test can provide further confirmation if needed.
If you test negative on a rapid test, it means that you are currently very unlikely to be contagious. A viral load that is too low to be detected by rapid antigenic tests is almost certainly too low to be transmissible. But it’s important not to let your guard down completely. The tests do not detect 100% of infectious cases, so it is possible that a small number will go unnoticed or that some cases will become infectious within hours of testing. For this reason, it may be wise to maintain other precautions. And, if you have symptoms or known exposure, it’s a good idea to do a rapid antigen or PCR follow-up test just in case the first test is a false negative.
Think of rapid antigen testing as a snapshot in time: A negative test doesn’t necessarily mean you don’t have COVID-19. COVID-19 is most transmissible when the viral load peaks, which is estimated to be less than a week after infection. Those who are infected but take a rapid test before or after the viral load peak will have a negative rapid test result – meaning that even if they are infected, they are not currently contagious. One way to reduce the risk of false negatives is to use “serial tests,” where a second rapid test is performed 24 to 36 hours later to help detect any infectious cases that were missed on the first test.
Will the new initiatives suffice?
White House initiatives to increase access to rapid tests are a crucial step towards reducing the number of cases. But one free test per person is not enough to help people safely return to normal activities. Allowing additional inexpensive rapid tests through the Food and Drug Administration would further increase supply and reduce prices.
Making the COVID-19 vaccine free and easily accessible saw cases drop rapidly in the spring of 2021. Making frequent rapid tests available to everyone could do the same now.
[The Conversation’s most important coronavirus headlines, weekly in a science newsletter]
This article is republished from The Conversation, a nonprofit news site dedicated to sharing ideas from academic experts. It was written by: Zoë McLaren, University of Maryland, Baltimore County.
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Zoë McLaren does not work, consult, own stock or receive funding from any company or organization that would benefit from this article, and has not disclosed any relevant affiliation beyond her academic position.
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