Coronavirus

Corona rapid tests: the antigen method as an alternative to PCR tests

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The result is presented like a pregnancy test: lines in a small window on a membrane strip show in a few minutes whether the result is positive or negative. A quick test that detects a corona infection in a very short time – that sounds like the next great hope. Used in companies or before major events, it could bring us back another piece of normality.

But how do these tests work? Are You Really Safe? And can they replace the tests commonly used up to now? Here are the most important questions and answers about the new antigen tests – mostly referred to as rapid tests.

What is the difference to the previous test method?

A so-called PCR test is currently used to test for a corona infection. A smear is taken from the throat or nose. A laboratory then examines this for bits of genetic material from the virus. According to the Robert Koch Institute (RKI), the actual test time is between four and five hours. However, it can take one to two days or even longer from the sampling to the result – depending on the laboratory capacity.

In contrast, a rapid test is an antigen test. For this purpose, doctors also evaluate a smear from the nose and throat area. Unlike the PCR method, the test does not look for genetic material, but for the proteins typical of the virus. Instead of several days, the result is available after a few minutes.

The sales company for life sciences Biozol from Eching near Munich already offers such a test in Germany. This is obtained from the company Rapigen from South Korea, one of the leading manufacturers of in-vitro diagnostics. Similar to the pregnancy test, the test consists of a membrane strip. If the pathogen is present in a sample, a visible black band appears in the result window. A control line will always appear if the test has been performed correctly.

The Swiss pharmaceutical giant Roche recently announced that it will also introduce an antigen test in Germany by the end of September. This should recognize in a maximum of 15 minutes whether someone is infected with Sars-CoV-2.

How does the approval process?

Unlike a drug, the corona test as a medical product does not have to go through an official approval process in Europe. Instead, a so-called conformity assessment procedure is used, at the end of which is the CE mark. In this, according to the Federal Institute for Drugs and Medical Devices (BfArM), equivalent to an approval procedure, the manufacturer must prove that his product is safe. In addition, the technical and medical services must be fulfilled exactly as they are described. As soon as a test belonging to the in-vitro diagnostics category is brought onto the German market, it is notifiable. The BfArM provides an overview of the tests on the novel coronavirus that have been displayed so far. The rapid test sold by Biozol is CE-certified and can be found in this list.

What are the advantages and disadvantages of the rapid test?

As the name suggests, the clear advantage lies in the speed of the test. “The result can be read after around eight minutes,” says Jonas Bäuerle, Managing Director of Biozol, in an interview with NewsABC.net. Another advantage is the low cost of this method. A fully equipped laboratory is necessary for the PCR test. All that is needed for the quick test is medically trained staff who know how to take the smear. “This ensures that the sample also contains enough virus material for the investigation,” says Bäuerle. An antigen test is therefore not suitable for use at home. A rapid test from Biozol costs around 15 euros. For comparison: a PCR test is currently around 50 euros.

The crux of the matter is the sensitivity, i.e. the reliability of the antigen method. The higher the sensitivity, the more reliably a test will detect an infection. Manufacturers indicate a high level of accuracy. The pharmaceutical company Roche speaks of a sensitivity of up to 96.5 percent: This means that out of 100 people tested, a good four receive a false negative result – that is, the test shows them as negative even though they are infected. The US company Abbott stacks a little higher and claims their antigen test has a sensitivity of 97.1 percent.

Biozol gives a sensitivity of 90.2 percent for the test they sell. In a clinical study carried out by the manufacturer, five rapid tests from 51 test subjects who tested positive for PCR were negative – i.e. different. In return, another important measured value, the so-called specificity, was very good. This indicates that too many percent of the test also recognizes a healthy person as healthy. Of 136 patients who tested negative for PCR, 136 antigen tests were also negative. The specificity would therefore be 100 percent.

When is an antigen test suitable?

Experts are currently investigating whether antigen tests could complement or even be an alternative to the PCR test. According to the Federal Ministry of Health, antigen tests show a slightly lower sensitivity and specificity than the PCR method. “In addition, they require more operational effort by the medical staff on site,” said press officer Oliver Ewald when asked by NewsABC.net. Whether an antigen test is suitable depends on the case: Does the patient have symptoms? Is it tested in a practice or on site? How soon is a result needed? If the antigen test gives a positive result, this should be reported to the health department in any case.

At the request of NewsABC.net, the RKI did not give an assessment and instead referred to the institute’s homepage. There the institute refers to a report from the World Health Organization (WHO) that dates back to April. In this study, the WHO still advises against antigen testing outside of research projects because of insufficiently assessable performance. The German Center for Infection Research, the German Society for Epidemiology and the Society for Virology were also unable to give an assessment of the antigen tests at short notice.

Does it make sense to use these tests even though they are less precise?

The antigen tests could help break the chains of infection by allowing a large number of people to be tested – even if they do not show any symptoms. Bäuerle names large events, old people’s and nursing homes and companies as examples. “The antigen test could also prove to be useful for travelers returning home.”

Mainly because the number of tests is increasing enormously. The RKI registered more than a million PCR tests in the last week of August. The laboratories therefore warn that a quick adjustment of the nationwide test strategy is necessary. There is a lack of consumables, the laboratory staff are working on the attack. Methods that are faster and use fewer resources than the PCR test are therefore urgently needed.

The decision of the 93rd Conference of Health Ministers of the Federal Ministry of Health shows that antigen tests are definitely being considered as an alternative. “New, high-quality antigen tests for Sars-CoV-2 can offer an important addition to the diagnostic options,” it says. Antigen tests would meanwhile achieve a high level of sensitivity and specificity.

For example, in the case of travelers returning without symptoms, the rapid tests can largely rule out an infection if the result is negative. Positive antigen tests, in turn, could be confirmed again using a PCR test. The Federal Ministry of Health, together with the RKI and the BfArM, intends to present a status report on the availability, quality and conceptual use of antigen tests by mid-September.

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