Fast coronavirus tests: what they can and can’t do
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Publicado por : dnfsdd85
Publicado en : 14-10-21
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Fast coronavirus tests: what they can and can’t do
The United States leads the world in COVID-19 deaths but lags behind many countries —
both large and small — in testing capacity. That could soon change.
At the end of August, the US Food and Drug Administration (FDA) granted emergency-use
approval to a new credit-card-sized testing device for the coronavirus that costs US$5,
gives results in 15 minutes and doesn’t require a laboratory or a machine for processing.
The United States is spending $760 million on 150 million of these tests from health-care
company Abbott Laboratories, headquartered in Abbott Park, Illinois, which plans to ramp up
production to 50 million per month in October.
The tests detect specific proteins — known as antigens — on the surface of the virus,
and can identify people who are at the peak of infection, when virus levels in the body are
likely to be high. Proponents argue that this could be a game changer. Antigen tests could
help to keep the pandemic at bay, because they can be rolled out in vast numbers and can
spot those who are at greatest risk of spreading the disease. These tests are also a key
element in the testing strategies of other countries, such as India and Italy.
Antigen assays are much faster and cheaper than the gold-standard tests that detect
viral RNA using a technique called the polymerase chain reaction (PCR). But antigen tests
aren’t as sensitive as the PCR versions, which can pick up minuscule amounts of the SARS-
CoV-2 virus that causes COVID-19.
This difference raises some concerns among specialists, who worry that
covid-19 antigen tests
will miss infectious people and result in outbreaks in countries that have largely
controlled coronavirus transmission. Others view the lower sensitivity as an attribute,
because some people who receive positive PCR test results are infected, but are no longer
able to spread the virus to others. So antigen tests could shift the focus to identifying
the most infectious people.
At present, antigen tests are administered by trained professionals, but some companies
are developing versions that are simple enough to be used at home — similar to pregnancy
tests.
“Making the tests faster, cheaper, easier is definitely the goal — and I think the
antigen test is the way to get there,” says Martin Burke, a chemist at the University of
Illinois at Urbana-Champaign, who is co-developing rapid tests, including antigen-based
assays. “This is by no means the perfect solution, it’s just the fastest thing we could
get going now,” he says.
What tests are there and how do they work?
Tests for COVID-19 fall into two categories: diagnostic tests such as PCR and antigen
assays, which detect parts of the SARS-CoV-2 virus, and
covid-19 antibody tests that
sense molecules that people produce when they have been infected by the virus. Antibodies
can take several days to develop after an infection and often stay in the blood for weeks
after recovery, so covid-19 neutralising antibody tests have limited use in
diagnosis (see ‘Catching COVID-19’).
The high-sensitivity PCR tests are almost 100% accurate in spotting infected people,
when they are administered properly. But such tests generally require trained personnel,
specific reagents and expensive machines that take hours to provide results.
Countries such as South Korea and New Zealand have succeeded in boosting PCR-based
testing, but scaling up these tests has proved difficult elsewhere. The United States, for
example, has seen a slow and poorly coordinated response to outbreaks, faulty tests from
the Centers for Disease Control and Prevention (CDC) and problems with the supply chain.
All of this has hindered efforts to collect and process samples for PCR, pushing waiting
times to days or even weeks. These delays, along with a lack of tests, have contributed to
the rampant spread of COVID-19 across the country, which by 16 September had seen almost
200,000 deaths from the disease.
A typical antigen test starts with a health-care
professional swabbing the back of a person’s nose or throat — although companies are
developing kits that use saliva samples, which are easier and safer to collect than a swab.
The sample is then mixed with a solution that breaks the virus open and frees specific
viral proteins. The mix is added to a paper strip that contains an antibody tailored to
bind to these proteins, if they’re present in the solution. A positive test result can be
detected either as a fluorescent glow or as a dark band on the paper strip.
Antigen tests give results in less than 30 minutes, don’t have to be processed in a
lab and are cheap to produce. Yet that speed comes with a cost in sensitivity. Whereas a
typical PCR test can detect a single molecule of RNA in a microlitre of solution, antigen
tests need a sample to contain thousands — probably tens of thousands — of virus
particles per microlitre to produce a positive result1. So, if a person has low amounts of
virus in their body, the test might give a false-negative result.
When used on people who were positive for SARS-CoV-2 in a standard PCR test, Abbott’s
antigen assay correctly spotted the virus in 95–100% of cases if the samples were
collected within a week of the onset of symptoms. But that proportion dropped to 75% if
samples were taken more than a week after people first showed symptoms. The sensitivity —
or the rate of detecting infections correctly — of the other antigen tests used in the
United States is between 84% and 98% if a person is tested in the week after showing
symptoms.
Companies and academic research labs are also rolling out other tests that are faster,
cheaper and more user-friendly than standard PCR assays, although they are not being
produced on the same scale as antigen tests. Some of
these other tests use the gene-editing tool CRISPR to zero in on genetic snippets of the
coronavirus. Others are quicker variants of the PCR test that use different reagents,
meaning they’re not limited by the same supply-chain problems. Saliva-based PCR tests, for
example, are being used as screening tools in universities and for professional basketball
teams.
Which tests tell whether someone is infectious?
Although the PCR method can test whether someone is infectious, it also detects people
who have the virus but are not likely to spread it.
Antigen-based testing, by contrast, could help to rapidly identify people who have high
levels of virus — those who are most likely to be infectious to others — and isolate them
from the community, says Marion Koopmans, a virologist at the Erasmus University Medical
Centre in Rotterdam, the Netherlands. “The question is, what is the safe limit? Because
the moment you get that wrong, the whole idea implodes,” she says. It’s still unclear
what viral load is the threshold below which a person is no longer contagious, says
Koopmans, who is working with the World Health Organization (WHO) to determine a standard
to validate rapid tests. “It would be very worrying if everyone does that on their own,
using different criteria,” she says.
Viral load peaks early in SARS-CoV-2 infections and then gradually declines, with tiny
amounts of virus RNA staying in someone’s nose or throat for weeks or possibly months2.
And although there are not enough data to equate different viral levels with how infectious
people are, there is evidence that individuals are unlikely to spread the virus about eight
to ten days after showing symptoms3.
“If you’re at risk of transmitting the virus to somebody else, you’re going to have
plenty of viral particles — those would certainly show up in antigen tests,” says Michael
Mina, an infectious-disease immunologist at the Harvard T.?H. Chan School of Public Health
in Boston, Massachusetts, who has been a vocal proponent of antigen tests.
There are challenges at the start of the infection, when people have low levels of the
virus. The answer, says Mina, is frequent testing — done multiple times per week. This
could quickly identify infected people, even if the assays are less sensitive than a PCR-
based test, because the amount of virus in their noses and throats rises within hours, he
says.
Mina and his colleagues have used statistical models to assess this strategy. In a
preprint updated on 8 September, they suggest that testing people twice a week with a
relatively insensitive test could be more effective at curbing the spread of SARS-CoV-2
than are more-accurate tests done once every two weeks1. Another study that modelled
different scenarios for safely reopening university campuses reported similar findings4.
There are also fertility tests.
To slow outbreaks, the focus should be on identifying those who are at risk of
spreading SARS-CoV-2 to other people, rather than on spotting anyone who is infected with
it, some experts say.
When used as a screening tool to frequently assess as many people as possible, rapid
antigen tests could be “a game changer”, says Rebecca Lee Smith, an epidemiologist at the
University of Illinois.