Professor of Microbiology Ath. Tsakris: Answers the questions about the reliability of the tests and how they react to Omicron

The professor answers the questions that arise and monopolize the interest in the last days.

Copy of Main Image x2 31 rapid test, Omicron

Professor of Microbiology and Vice Rector of the National and Kapodistrian University of Athens (EKPA), Athanasios Tsakris, intervenes in the discussion about the reliability of rapid coronavirus detection tests, but also about their effectiveness against the Omicron variant. In an article in Kathimerini, the professor answers the questions that arise and monopolize the interest in recent days.

According to the tests, molecular and antigenic, are among the basic tools we have to manage the pandemic. As long as we know the range of limitations they have, but also how their results are interpreted. And especially to realize that with rapid and self antigen antigen tests we sometimes sacrifice the accuracy for the speed and convenience that they undoubtedly provide us. On the other hand, the correct application of antigen tests is an indicator of infectivity: repeated negative results with a reliable rapid test show that the chances of transmitting the virus are now limited. So let's look at some of the parameters of the correct use and interpretation of all tests.

How do molecular tests work and how do rapid tests work?

Molecular tests (PCR) detect the genetic material, RNA, of SARS-CoV-2 and in fact in its extremely low concentration. They are done by nasopharyngeal or oral pharyngeal mucosal smear. Rapid antigen tests detect antigens of the virus, ie small viral proteins, and are done mainly by smearing the nasopharynx.

How sensitive are different types of tests to detecting the virus?

In molecular tests, the chances of a false result are minimal. Their accuracy is close to 100% when the right laboratory practices are followed and they can detect the virus even in asymptomatic individuals with very low viral load. Antigen tests are less sensitive and can detect active infection mainly in the first days of infection.

In particular, according to an analysis of recent studies, they give a more accurate positive result (on average 78%) when they occur during the first week of symptoms, when the viral load is usually very high.

In the second week this percentage is reduced to 50%. Precisely because they require a relatively high viral load to indicate whether a person is positive for SARS-CoV-2, their accuracy is limited to asymptomatic individuals: they detect the virus at a rate not exceeding 60%.

Does build quality play a role in the reliability of a test?

Of course. This applies to all tests, but especially to rapid and self tests. Analysis of many recent studies confirms that their sensitivity varies greatly depending on the test company: it ranges from 35% to 90%!

What else ensures the accuracy of an antigen test?

The decisive factor is the correct taking of the sample - more so than in the molecular test - based on the instructions. It is important, that is, to have a sufficient amount of nasopharyngeal smear on the swab so that the method can detect the virus antigen. An inadequate sample greatly reduces the chances of having a positive result. Proper use, the range of limitations they have and how their results are interpreted.

Does the thickness of the line in the positive rapid test indicate a high viral load and high infectivity?

Antigen tests are not designed to assess the viral load present in the sample being tested. Nevertheless, theoretically always, a thick line in the positive test may be in favor of an increased infectivity.

Can a rapid test give a false positive result?

Under normal circumstances, a reliable antigen test will rarely give a false positive result, that a person is positive for the virus, that is, when in fact it is not. In any case, the result can be confirmed by a molecular test.

Does a positive molecular test followed the next day by a negative one mean that the first result was false positive?

Not necessarily. In many cases, a sufficiently sensitive molecular method can detect even a low viral load, but this may be undetectable in a subsequent test.

Is the "Omicron" variant detected by the rapid tests on the market?

Experience to date has shown that a reliable antigen test can detect the "Micron" variant, as well as previous variants. However, according to recent data, at the beginning of the infection, when the viral load is still low, the existence of this variant may escape rapid tests. Thus, when we experience symptoms compatible with COVID-19, let's not rest because of a negative antigen test.

This is, after all, the logic of rapid tests: their frequent and repeated use. In any case, let us keep in mind that due to the extremely high transmissibility of "Omicron", the negative test results actually "expire" earlier. After all, every molecular or antigen test is a recording of a single moment: the moment the pen enters the nose to be sampled. That is why protective measures should not be relaxed due to the feeling of security from a negative result.