How reliable are rapid antigen tests? | Science | In-depth reporting on science and technology | DW | 01.12.2021

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How reliable are rapid antigen tests?

Rapid antigen tests for detecting the coronavirus could be giving people a false sense of security. In Germany, manufacturers are allowed to certify the efficacy of their tests without independent quality control.

Two negative tests and the Brandenburg Gate in Berlin

People should still be careful even if they test negative

Rapid antigen tests are playing an increasingly important role in the fight against the coronavirus, not only for people who are not vaccinated but also for those who are. Tests are being conducted at home and at work, in private and in public. In Germany, some places have introduced the 2G+ rule, which means that as well as providing proof that they are either vaccinated or recovered from COVID-19, people also have to provide a recent negative COVID-19 test.

Therefore, many people are relying on rapid antigen tests, which were approved for private use at the beginning of 2021 because they can detect infection early on and interrupt the chain of infection. They are often used before people pay a visit to older or vulnerable relatives in care homes or before small gatherings with friends and/or family.

A positive test

Experts recommend getting a PCR test if an antigen test shows positive

However, they can give people a false sense of security. In a study conducted by researchers at Germany's Paul Ehrlich Institute, the Robert Koch Institute and Berlin's Charite Hospital, 20% of the antigen tests available on the market failed to detect the virus even when there was a high load.

According to the preprint, 26 of the 122 examined tests did not even meet the minimum requirement, which is 75% sensitivity. Rapid tests should work in 75% of cases, if a sample contains a defined amount of the virus. This amount is determined by the cycle threshold (Ct) value, which is the number of cycles it takes to detect the genetic material of SARS-CoV-2 in a sample. The lower the Ct value, the higher the viral load. If it is 25 or below, the viral load is very high, meaning that a person is very contagious. If it is between 25 and 30, the viral load is high. Between 30 and 36, it is moderate.      

About 80% of the tests detected the virus when the load was very high and 60% detected all the cases of highly contagious samples. But 20% still failed to detect the virus.

Antigen tests are less reliable than PCR tests

Antigen tests and PCR tests both involve taking a sample of secretions from mucosal surfaces, through the nose and/or the back of the throat.

The antigen tests examine these secretions for certain proteins in the viral envelope. If a sample only contains a small amount of the virus, for example if a person is at the beginning or end of an infection or because they drank a lot before taking the test, then such tests are not very reliable.

If a person tests positive, they should definitely also have a Polymerase Chain Reaction (PCR) Test,which is more accurate and able to detect very small amounts of the virus.

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Manufacturers certify quality themselves

Surprisingly, manufacturers are currently allowed to sell rapid antigen tests without external quality assessment and issue the CE mark of compliance with EU safety and quality regulations themselves.

"Pursuant to the currently applicable EU Directive for in vitro diagnostic devices (IVD), tests for the human immunodeficiency virus (HIV) rank among the category of high risk products and are therefore subject to the certification by a Notified Body," explains the Paul Ehrlich Institute on its website. "COVID-19 tests are currently still categorised as "low-risk IVDs" and as such, can be certified by the manufacturers themselves. They do not require independent testing before they can be marketed."

This unacceptable state of affairs is slated to continue until May 2022, the Paul Ehrlich Institute continues: "As of that point in time, as per the IVD Regulation an EU reference laboratory and a Notified Body will have to be consulted for their certification, as the COVID-19 tests will come under the highest risk category. In future, this will require laboratory testing of the tests as well as independent verification of the data."

A COVID-19 testing center in Duisburg

All over Germany, people are queuing up for COVID-19 tests

Antigen tests are still helpful

Does that mean that rapid antigen tests are pointless? No! They can help to contain the pandemic by detecting infection fast and thus breaking the chain of infection. They raise the alarm if somebody has a high viral load and is therefore very contagious.

However, people have to be aware that a negative test does not necessarily mean that somebody does not have the infection and is not contagious. People can be infected and contagious for two days before the first symptoms even manifest themselves. Some might not show any symptoms at all.

So beware that false sense of security!

This article was translated from German.