Covid-19 rapid tests in comparison - what you need to know before buying

Are you looking for a rapid coronavirus test? There are currently so many different products on the market that it's easy for laypeople and even doctors to lose track. In this article, you will learn 5 important things you need to know about coronavirus rapid tests.

1. difference between PCR test, IgG & IgM antibody test and antigen tests

If you don't know what the difference is between a PCR test and a Corona antibody test or Corona antigen test then you've come to the right place. A PCR test according to a certain standard is currently recognized by most governments. A PCR test requires trained specialists and expensive equipment that has to be maintained on a daily basis. This means that a PCR test is complex and expensive, but offers slightly higher accuracy (sensitivity) compared to antibody tests. Sensitivity refers to the ability of a test to correctly identify the infected persons.

The sensitivity of standard PCR tests is around 99% - 99.9%. A PCR method can therefore "find" a positive patient with a high probability, even shortly after they have been infected with coronavirus.

With the IgG / IgM antibody test requires a drop of whole blood, which is applied to a test cassette. The test result can be read within a few minutes, but the sensitivity is "only" 97%-98%. This means that 2-3% of infected persons are not "found" with the antibody test. An antibody test is positive just a few days after infection and indicates a positive immune response to the coronavirus.

Corona antigen rapid tests such as the new test from Roche for example, are already positive shortly after infection with the SARS-CoV-2 pathogen, as they do not detect the body's immune response but viral protein. Therefore, this test is already positive at the beginning of an infection, similar to a PCR test. Below you will find a graphic that illustrates this.

2. why does even a PCR test not have an accuracy of 100%?

There is no such thing as 100% accuracy. Just as there is no 100% certainty that you will survive today or that no plane will crash today. If, for example, you were infected with the coronavirus just a few hours ago, a PCR test would not (yet) be able to detect your early course of infection. Therefore, the accuracy is about 99%, but not 100%.

Click here for an overview of all antigen and antibody rapid tests!

How does a PCR test work?

The polymerase chain reaction (PCR) is a widely used method for rapidly producing millions to billions of copies of a specific DNA sample, allowing scientists to take a very small DNA sample and amplify it into a large enough quantity to study in detail. The PCR method was invented in 1984 by the American biochemist Kary Mullis at the Cetus Corporation. It is fundamental to much of genetic testing, including the analysis of old DNA samples and the identification of infectious agents. PCR is used to exponentially amplify copies of very small amounts of DNA sequences in a series of cycles of temperature changes. PCR is now a widely used and often indispensable technique used in medical and clinical laboratory research for a variety of applications, including biomedical research and criminal forensics.

The majority of PCR methods are based on thermal cycling. Thermal cycling exposes the reactants to repeated heating and cooling cycles to enable various temperature-dependent reactions - in particular DNA melting and enzyme-driven DNA replication. PCR uses two main reagents - primers (which are short single-stranded DNA fragments known as oligonucleotides that represent a complementary sequence to the target DNA region) and a DNA polymerase. In the first step of PCR, the two strands of the DNA double helix are physically separated at high temperature in a process called nucleic acid denaturation. In the second step, the temperature is lowered and the primers bind to the complementary sequences of the DNA. The two DNA strands then become templates for the DNA polymerase, which enzymatically assembles a new DNA strand from free nucleotides, the building blocks of DNA. In the course of PCR, the generated DNA itself is used as a template for replication, which initiates a chain reaction in which the original DNA template is multiplied exponentially.

Why do you need a nasal swab for the PCR test?

In order to detect DNA in a polymerase chain reaction, sufficient cells must first be obtained. Cells are therefore taken from suspected infectious mucous membranes such as the nose, throat or oral mucosa using cotton swabs. These cells are multiplied in a laboratory as described above so that the DNA virus particle can then be searched for. You therefore have no choice but to donate your cells using a cotton swab. Without genetic cell material, a PCR test is therefore not possible.

3. how does a corona antibody test or corona antigen test work?

An immunoassay is a biochemical test that measures the presence or concentration of a macromolecule or small molecule in a solution by using an antibody or antigen. The molecule detected by the immunoassay - in our case the virus itself - is in many cases a protein, although it can also be other types of molecules of different sizes and types, as long as the right antibodies with the appropriate properties for the assay are developed. For example, viruses that may be present in biological fluids such as blood serum or urine are often measured using immunoassays or antigen or antibody tests for medical and research purposes.

You therefore need to take a drop of whole blood from your fingertip using a Lancette and apply it to the test cassette together with the carrier solution. The test result is displayed visually after just 15 minutes. The sensitivity is 97-98% depending on the manufacturer.

Click here for an overview of all antigen and antibody rapid tests!

4. how long do I have to be infected for rapid tests to be positive?

There are different types of COVID-19 tests. Some directly detect the DNA particles from the SARS-CoV-2 virus itself, others look for an immune response of your body to a current or past infection. The likelihood of a positive test result depends primarily on the correct choice of test, depending on when the infection with SARS-CoV-2 occurred. The following diagram illustrates when PCR testing makes sense, when Rapid tests can detect an infection and when Antibody tests should be used:

PCR tests can detect even the smallest virus DNA particles at an early stage and remain positive for a long time, even if the patient is no longer infectious.

Rapid tests detect viral proteins, for example, and are positive when a patient is most infectious. This type of rapid test is known as an antigen test. The new Roche SARS-CoV-2 antigen test is used for the qualitative detection of SARS-CoV-2 antigens. This test has been available since October and is packaged in 25 pieces.

Antibody tests detect a patient's immune response and are therefore only positive after a few days. Immunoglobulin M antibody tests detect an active immune response, immunoglobulin G antibody tests detect an immune response that has already expired. 

For the professional sector, we offer the CLINITEST Rapid Antigen Test Professional from Siemens which delivers very high-quality results.

5 Which test should I buy now?

You need to ask yourself whether you want to detect an acute infection in the first week after infection or an infection that has already been present for weeks.

Do you want to know whether you have just been infected with SARS-CoV-2?

Then you need to buy a rapid antigen test. This is packaged in 25 pieces and supplied by the well-known pharmaceutical company Roche offered. Here you can order the rapid antigen test from the company Roche for the equivalent of € 3.56 per test.

Do you want to detect an infection that has been present for weeks?

Then we recommend IgG / IgM antibody tests. We have 3 high-quality tests from leading pharmaceutical companies to choose from

• Green Spring® Antigen 4 in 1 rapid test (0,20 € per test)
• Siemens CLINITEST Rapid COVID-19 rapid test (0,95 € per test)
• Roche Antigen test (3,56 € per test)

Click here for an overview of all antigen and antibody rapid tests!

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