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Coronavirus: What You Should Know About Testing

What corona tests are there?

More and more tests for the coronavirus are coming onto the market. By mid-November there were already more than 400 different products around the world. These can essentially be divided into three main groups: PCR tests, serological tests (ELISA) and antigen tests.

PCR tests

In order to check whether someone is currently infected and can also infect others, a test using the polymerase chain reaction (PCR) method has usually been used so far. Alternatively, there are also so-called isothermal DNA amplification tests, which, however, work very similarly to PCR tests.

In both cases, saliva is removed from the patient's throat using a cotton swab. Ejection from the deeper lungs is also suitable for detection. Then a certain part of the genetic material - such as a gene - is reproduced from the sample in numerous steps and at the end, a biochemical process called agarose gel electrophoresis is used to determine whether the expected virus genetic material was contained in the sample.

Usually the samples are taken from the throat or from the nose.

If the genetic material is found, the patient is potentially considered to be infectious (a doctor then has to assess the viral load). If the genome is not found, however, that does not necessarily mean that the test person is not infected after all. It could still be that the viruses weren't in the sample but are present elsewhere in the body.

This may also explain why in individual cases in COVID-19 patients who were already considered cured, later PCR tests turned out positive. In these cases, the viruses were likely to have been present all the time but were not found in the pre-discharge test.

Charité infection medicine specialist Christian Drosten compared this in his NDR podcast with trying to catch a goldfish in a basin with a net. If you pull the net out of the water and there is no fish in it, it does not mean that there are no fish in the pool.

Rapid PCR tests

Traditional PCR tests need to be done in a laboratory. Usually this happens in high throughput, whereby thousands of samples can be tested at the same time. However, this usually also means that the patient receives his findings after many hours at the earliest, and usually not until days.

It could go faster with so-called PCR rapid tests. The test is not carried out in a central laboratory, but can be carried out on site using mobile devices. The analysis process of these devices can be optimized so that a result is available within three quarters of an hour. The disadvantage: the devices do not manage more than 80 tests a day.

Antigen tests 

These new rapid tests have only been on the market this summer - and according to some manufacturers, they should be as easy to use as a pregnancy test.

But that's not quite the case with all products. Unlike the pregnancy test, most rapid tests still require a small mobile analysis device to prepare the sample for virus detection using the fluorescent immunoassay (FIA). So not all of them are suitable for domestic use.

A few products can do without an analyzer, but that's not the only reason why these rapid tests are not freely available. Because in Germany, for example, all SARS-CoV-2 diseases are notifiable under the Infection Protection Act. Therefore, only doctors should perform the tests.

Many of the tests now achieve good sensitivity and, above all, a sufficiently high specificity of over 98 percent, as the German member of the Bundestag and physician Karl Lauterbach emphasized in a tweet.

This means that there are few false positives. The research team of the virologist Christian Drosten from the Berlin Charité had compared seven tests with each other, of which only two had failed. These had only achieved a specificity of between 88 and 94 percent. The results were published as an unreviewed preprint on MedRXiv on November 13th.

The sample is also taken from the saliva for the antigen test. The tests should provide the result within fifteen minutes of whether a patient is acutely infected and contagious. The advantage is the fast availability and the possibility to use it directly on site.

Flu tests that work on the same principle achieve a sensitivity of a little more than 50 percent and a specificity of around 99 percent. That would mean that only every second virus carrier is actually recognized. One in 100 people tested would have a false positive result.

Nevertheless, more and more doctors are in favor of the widespread use of antigen tests, because they hope that more infected people can be discovered in this way, because the tests are particularly effective when the patients are currently under a high viral load. This is also the moment when they are most likely to infect others as super spreaders.

Evidence of past infection

Serological tests (ELISA) detect antibodies in the blood of the test subjects that the immune system has raised against the virus. This means that the body has already shown an immune response to the infection with a corresponding virus. To do this, the test person has to submit a small blood sample. This is tested in the laboratory.

More on this: The immune system in the fight against corona

Manufacturers now also offer rapid tests based on this principle, which, however, have to be carried out by a practicing doctor. All you have to do is drop a few drops of blood - similar to a diabetes test - into a test cassette and add a buffer solution.

If the SARS-CoV-2 typical immunoglobulins IgM and IgG are in the blood, the sample changes color. A positive result can mean that the test person has had a corona infection and now has a certain immunity to it. But it doesn't have to be like that. Because almost all antibody tests "react cross-wise", as Drosten emphasized in his podcast.

On the other hand, some manufacturers assert that this is not the case with their products. In any case, it is conceivable that someone who tested positive has had another coronavirus infection - such as a cold - but not an infection with SARS-CoV-2.

When does a test make sense and for whom?

PCR and antigen tests are important to find out in patients and their contact persons whether they are currently infectious and what form of quarantine they need to be in: Is it enough to give a contact person a two-week curfew, during which they can still meet members of the household , or does it really have to be sealed off?

A positive antibody test can mean that the subject has had a corona infection. But it doesn't have to be like that.

ELISA tests are an important indicator for epidemiologists in order to be able to estimate how many people have gone through an infection undetected and whether a certain herd immunity will possibly be achieved at some point. This can help politicians decide on easing restrictions.

The test can also help to check people who have definitely contracted COVID-19, or those who have received one of the newly developed vaccinations, for their immunity.

Various German university hospitals have started large-scale studies in which they use ELISA tests to check randomly selected participants for a possible undetected infection - also to find out more about the behavior of the virus.

Test strategies from different countries

Countries around the world are handling corona tests very differently in the course of the current pandemic. There are many reasons for this: differences in the performance of health systems, the availability of tests and different laboratory capacities played just as important a role as the question of how seriously the threat was taken from the start.

South Korea's drive-in corona test also quickly caught on worldwide

South Korea, for example, which had learned from the experience of the SARS epidemic of 2002, was one of the countries that systematically tested many people very early on, even those who had no symptoms - even when the case numbers were still comparatively low.

With regard to the total population, Germany is also one of the countries that test a lot, but mainly people who have demonstrably had contact with infected people and show symptoms. Other countries - such as the USA - are currently massively expanding their test capacities, but the pandemic there is also more advanced and the number of cases is very high. Against this are states in Africa, where there is hardly any testing.

This article was last updated on December 11th, 2020 due to new information since it was first published.

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