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Rapid antigen coronavirus tests are just as accurate as laboratory tests when used every three days, a new study from the National Institutes of Health (NIH) finds.
Both test types caught 98 percent of COVID-19 cases, the researchers found through regularly testing college students and staffers at the University of Illinois.
Antigen tests detect proteins found on the surface of the virus, rather than genetic material, which is what laboratory, or polymerase chain reaction (PCR), tests look for.
PCR tests have traditionally been considered more accurate at identifying cases when a patient is further along in their disease progression.
However, swabs need to be sent to a laboratory and take two to three tests for results to return where as rapid antigen take just 15 minutes to return.
When test frequency goes down to once a week, researchers found antigen tests catch only 80 percent of cases.
The study suggests that antigen tests may be invaluable in schools, workplaces, and other settings during future COVID-19 outbreaks.
Antigen tests are cheaper and produce results quickly – making them perfect for screening programs. Pictured: A woman gets swabbed for an antigen test in Lisbon, Portugal
![](https://i.dailymail.co.uk/1s/2021/07/01/18/44923847-9746227-image-m-27_1625161535782.jpg)
![When used frequently, antigen tests are just as effective as PCR tests, the NIH study found](https://i.dailymail.co.uk/1s/2021/07/01/18/44923929-9746227-When_used_frequently_antigen_tests_are_just_as_effective_as_PCR_-m-25_1625161508617.jpg)
When used frequently, antigen tests are just as effective as PCR tests, the NIH study found
If people need to get COVID-19 test, PCR tests are the gold standard.
These tests are highly accurate because they look for specific genetic material matching the coronavirus in a sample from someone’s mucus or saliva.
However, PCR tests are expensive and may be time consuming.
In the early months of the pandemic, some Americans who got PCR tests had to wait over a week for their results – and even now, wait times may be two days or more.
During that wait, the coronavirus can continue spreading undetected.
As a result, many public health experts promote antigen tests – rapid tests that look for proteins in the surface of the coronavirus.
Antigen tests are cheap, easy to administer, and can be performed in a doctor’s office, at school, or even at home – and results may be delivered in under 15 minutes.
Antigen tests come with a downside, though – they are not as accurate as PCR tests.
These tests have a lower sensitivity, meaning that those who get tested with antigen tests are more likely to be false negatives – receiving a negative result even though they’re actually infected.
However, past modeling studies have shown that, when antigen tests are used frequently, that repeated use will catch the majority of Covid cases in a community.
The NIH researchers conducted this study, with results published in The Journal of Infectious Diseases, at the University of Illinois, which was already using frequent testing to identify cases on campus.
When a student or staff member tested positive this past spring, they became eligible for the NIH study.
Those living in close contact with a person who tested positive – for example, the roommate of a student who got Covid – were also eligible.
In total, 43 students and staffers swabbed themselves for 14 days in a row during the course of their Covid infections.
![Antigen tests are more accurate near the start of a patient's Covid infection - when they are most likely to give the virus to someone else](https://i.dailymail.co.uk/1s/2021/07/01/18/44923925-9746227-image-a-19_1625159430877.jpg)
Antigen tests are more accurate near the start of a patient’s Covid infection – when they are most likely to give the virus to someone else
Researchers on-site at the University of Illinois ran both PCR and antigen tests on the samples.
They also sent samples to a lab at Johns Hopkins that did additional analysis, estimating the onset and duration of Covid infectiousness in each participant.
The researchers found that both the PCR and antigen tests were most accurate when patients were at their peak infectiousness or most likely to give the coronavirus to someone else.
As infectiousness faded, the antigen test became less accurate.
This test was less likely to return positive results later in a patient’s disease progression, while the PCR test still returned positive results every time.
Experts say that getting a positive result when a patient is most infectious is actually more important than catching their case later – since this earlier result is more likely to stop the patient from infecting others.
The NIH researchers used their results to estimate how accurate PCR and antigen tests would be when they’re used regularly.
At shorter intervals – every two or three days – both tests have 98 percent sensitivity, the researchers found. That means both tests will catch 98 percent of all Covid cases in a population.
At longer intervals, the PCR test remains highly accurate – at 98 percent sensitivity – while the antigen test becomes less accurate. At one week intervals, the antigen test sensitivity was only 80 percent.
![Antigen tests are most likely to catch cases when used frequently. Pictured: A patient gets swabbed in Portimao, Portugal](https://i.dailymail.co.uk/1s/2021/07/01/18/44923969-9746227-image-a-24_1625159621211.jpg)
Antigen tests are most likely to catch cases when used frequently. Pictured: A patient gets swabbed in Portimao, Portugal
This study confirms that antigen tests are extremely useful for regular Covid screening.
‘Rapid antigen testing at home, two to three times per week, is a powerful and convenient way for individuals to screen for COVID-19 infection,’ said Dr Bruce Tromberg, director of the National Institute of Biomedical Imaging and Bioengineering (NIBIB), in a statement.
‘With schools and businesses reopening, an individual’s risk of infection can change from day to day. Serial antigen testing can help people manage this risk and quickly take action to prevent spread of the virus.’
In fact, some universities and businesses are already using antigen tests to screen for Covid and prevent outbreaks.
The American Rescue Plan has provided funding for more K-12 schools to set up similar testing programs in the fall.
These programs will be key in preventing outbreaks as Delta and other variants continue to spread.
‘Silent transmission of the SARS-CoV-2 virus from individuals with no symptoms contributes significantly to the spread of the virus,’ said Dr William Heetderks, also from NIBIB and a lead author on the study.
‘Faster, cheaper and broader testing with antigen tests can be a big help in the kind of large-scale screening scenarios that can find these silent transmitters.’
![](https://i.dailymail.co.uk/1s/2021/07/01/18/44923899-9746227-image-a-23_1625159533007.jpg)
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