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Corona: Why PCR tests are reliable

2020-12-04T21:06:17.804Z


Corona skepticism is unleashed on hardly any topic as in PCR tests and Christian Drosten. Why? And what is it about the arguments?


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PCR tests: "The current numbers with seriously ill and many dead lead this discussion ad absurdum"

Photo: Thomas Lohnes / Getty Images

Closed restaurants, poor order books, ban on visits to hospitals: the corona crisis demands a lot from everyone in Germany, doctors, parents, politicians, carers, short-time workers.

Different rules apply depending on the state, time or street.

And what was true yesterday is not infrequently outdated tomorrow.

That is exhausting.

Again and again politicians and every single person in Germany have to balance how much freedom is still possible, which contacts are absolutely necessary and what is too much - so that everyone can get the best possible medical care should they become infected.

It is all the more justified to critically question measures.

Why are restaurants of all places being closed?

Why does only one parent have to be quarantined in certain cases when the child may have Corona?

One can discuss that.

But not every discussion is productive.

The basis of almost all political decisions on the restrictions is the current infection rate, i.e. the number of corona infected people newly reported every day.

These are recorded by the Robert Koch Institute (RKI) based on the results of the corona tests transmitted by the laboratories.

The tests are therefore an important measuring instrument for combating pandemics.

Again and again, a hard core of corona critics and online platforms stir up discussions about the reliability of the so-called PCR tests (

polymerase chain reaction

), which can detect a corona infection.

The criticism spreads particularly in social networks, also because it comes in part from scientists and doctors.

A selection of the arguments - and what is known about them.

Can the PCR test reliably detect a corona infection at all?

The PCR test reacts to certain sections in the genetic material of the virus that are

only found

in the Sars-CoV-2 coronavirus, which is causing the current pandemic.

Most tests show

two different sections of the genome

, some even three.

It is therefore extremely unlikely that the tests will also react incorrectly to other coronaviruses.

It is wrong to say that they are intentionally designed to respond to other viruses.

Even

if only gene segments are detected, the test is reliable

.

Because wherever parts of the pathogen are, the whole virus has very likely spread.

On the side of the Robert Koch Institute (RKI) it is clear: "An acute infection with SARS-CoV-2 is determined

by means of direct pathogen detection

(eg genome detection using PCR, antigen detection, virus isolation)."

So are the critics wrong?

There is quibbling behind the discussion about the detection of an infection.

The law defines an infection as "ingestion of a pathogen and its

subsequent development

or multiplication

in the human organism".

Because PCR tests do not show whether the detected virus components can still infect cells or are no longer infectious, the critics argue that, strictly speaking, the tests cannot detect an infection within the meaning of the Infection Protection Act.

As evidence for this, it is often cited that even the Berlin Senate is said to have confirmed this in an answer to the request of a member of parliament.

In fact, the answer was that PCR testing cannot prove whether someone is contagious.

But that is not secret knowledge that was disclosed there, but known.

The tests cannot tell for sure whether someone is contagious.

However, they do prove an infection.

 “If the PCR works, then the pathogen has multiplied.

Whether he is still doing this at the moment the sample is taken is irrelevant, "said virologist Friedemann Weber, director of the Institute for Virology at the Justus Liebig University in Giessen, the Correctiv research center.

According to the RKI, a positive PCR test for the coronavirus is enough to assume an acute infection within the meaning of the Infection Protection Act.

The Berlin Senate also sees it that way.

When asked why he made his corona measures against the corona pandemic dependent on PCR tests, the Senate replied: “Because the PCR test detects the presence of SARS-CoV-2 viruses.

The presence of these viruses correlates with infection with these viruses.

This infection is relevant to considerations about infection protection. "

How important are PCR tests in making quarantine decisions?

It is difficult to determine whether someone is contagious or not in the laboratory.

Scientists can examine whether viruses from certain samples can attack cells - but only under laboratory conditions.

This is time-consuming and it is not certain whether the viruses would also spread in reality.

However, observational studies of outbreaks have shown that

infected people can pass the pathogen on before they become sick themselves

.

That is also the reason why a mask requirement makes sense for everyone.

Because nobody can know whether he or she will get sick in a few days.

Anyone can carry the virus and should therefore protect others.

After the onset of symptoms, infected people remain contagious for an average of eight days.

It is all the more important to stay at home as soon as you suspect you are infected or have symptoms that suggest an infection.

And

don't wait for a positive test result

.

In any case, a positive PCR test is not absolutely necessary for ordering a quarantine.

In general, anyone who has had contact with someone who has been known to be infected for at least 15 minutes should be quarantined.

And the RKI recommends that anyone with symptoms of a respiratory disease stay at home for at least five days.

You can read here who should be tested.

The RKI recently shortened the quarantine time for contact persons of corona infected people to ten days if a negative test is available.

Are most of those who test positive not even contagious?

PCR tests can still be positive even if those affected are not yet or no longer contagious.

Some therefore argue that the high number of new infections detected says little because

most of

those who

tested positive

are not contagious anyway.

They refer to the so-called Ct value.

It indicates how often the components of the viral genome had to be replicated in the laboratory before the test worked.

If there is a lot of virus material in the sample, a few such copy cycles are sufficient.

However, if a sample has to go through more than 30 of these cycles, it probably does not contain any viruses that could infect cells under laboratory conditions, show initial analyzes.

This can mean that the person concerned only had a low virus load and may not be able to infect others.

In the USA, the proportion of tests with such high Ct values ​​was up to 90 percent, reports the New York Times.

Of 963 positive tests evaluated in Schleswig-Holstein, according to the Süddeutsche Zeitung, almost half had such high Ct values.

Critics claim that the tests are not meaningful with such high Ct values.

Laboratory workers searched for the virus genome in the samples until the tests were positive.

But that's not true.

Usually a maximum of 45 copy rounds are carried out.

And: A high Ct value does

not automatically mean that the person

from whom the sample originates is

not contagious

:

  • Even

    taking

    the

    sample can have a considerable

    effect on

    the Ct value

    .

    If only a small amount of material from the throat or nose lands on the swab, it is no surprise if only a small amount of the virus genome can be detected.

  • In addition, the Ct value only says something about the

    moment the sample was

    taken.

    It is therefore unclear whether someone could have been infectious before or will still be.

    The RKI believes that it is justified to release a demonstrably infected person from isolation due to a low Ct value if the sample was taken ten days after the onset of symptoms and the person affected was significantly better in the two days before the test.

"The scientific evaluation of the development of Ct values ​​is currently the content of various data evaluations," said laboratory physician Jan Kramer to SPIEGEL on request.

"A derivation with regard to the assessment of the pandemic development seems to me to be premature and only possible to a limited extent," said the board of the Association of Accredited Laboratories in Medicine.

What role do false positive results play?

Individual doctors and corona skeptics argue that PCR tests give false-positive results in one to two percent of cases.

This would incorrectly declare healthy people to be infected.

In fact, PCR tests should be much more reliable.

In New Zealand, for example, not a single test was positive from the end of May to mid-June, although up to 4,000 people in the country were checked for infection every day.

(Read more about this here.) If up to two percent of the tests really work, even though there is no infection, at least some of the results should have been positive.

In principle, however, no test is absolutely error-free.

In an Augsburg laboratory, for example, a noticeably large number of PCR tests were briefly positive.

It later emerged that the laboratory had used unsuitable reagents, probably because of supply shortages.

Nevertheless: "The method of PCR is an established method in molecular diagnostics and has been used routinely for many decades," the Society for Virology recently emphasized in a statement.

"The data reported by the RKI on the incidence of infection reflect medical findings and not raw test results," the statement continues.

Why is the criticism directed against Christian Drosten again and again?

The research team led by Christian Drosten from the Charité was the first to provide a test method for the coronavirus.

The PCR test could only be developed because genetic information about the virus was published in China.

The research team compared the data with tests already developed against the Sars coronavirus and coronaviruses in bats.

The researchers then tested patient samples that had been shown to have been infected with other coronaviruses and cold viruses in general.

All results remained negative.

So the test only responded to the new corona virus.

In theory, the test could also work with the Sars corona virus or bat corona virus, Drosten said on the NDR podcast.

The Sars virus has not existed in humans for 16 years.

The bat viruses do not occur in humans either.

"This test does not react against any other human coronavirus," said Drosten.

A hard core of critics continues to doubt the test.

A current report goes into great detail and accuses Drosten's team, among other things, of not having correctly abbreviated the unit nanometer at some points in the study.

Overall, according to the allegation, the study must be withdrawn.

If you ask around experts, you get the impression: You are tired of the constant discussions.

Hardly any laboratory is still using the January PCR test developed by Drosten's research team

.

Companies have long since developed their own tests that can be carried out quickly and in large numbers.

Only in this way are so many tests possible.

The whole discussion is superfluous.

"The current numbers with seriously ill and many dead take this discussion to absurdity," said virologist Ulf Dittmer from the Essen University Hospital.

"How can you still argue that the PCR only gives wrong results?"

Icon: The mirror

Source: spiegel

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