Types of tests

In the context of the corona pandemic, there are many different testing options to detect infection with the coronavirus. The following tests are currently being used in Luxembourg:

  • PCR test
  • Antigen test (rapid test and self-test)
  • Antibody test (serological test)

PCR test

The abbreviation PCR stands for Polymerase Chain Reaction and was developed by Kary Mullis in 1983. PCR is used in medical diagnostics, among other things, to detect various infectious diseases.

The PCR test is probably the best-known and most commonly used test since the beginning of the pandemic to detect infections with the SARS-CoV-2 coronavirus. The PCR test is considered the gold standard among the corona tests and is thus the most reliable method to detect an infection with the virus in symptomatic or asymptomatic persons.

How does it work?

The PCR test can be used to amplify specific DNA or RNA sequences outside the living organism. The genetic material of the virus is multiplied (amplified), which makes it possible to detect viruses even if the viral load is low.

The following components are required to carry out a PCR:

  • examination material (e.g. tissue samples)
  • specific reagents [nucleotides (DNA and RNA building blocks)] and laboratory equipment
  • a detection system for the PCR products

The swab is usually taken from the nose or mouth. Genetic material (RNA) of the SARS-CoV-2 virus can be obtained from here. In the laboratory, the genetic material is amplified in several cycles. Afterwards, so-called fluorescent molecules are used, which attach themselves to the genetic material. This way, it can be determined whether the specific gene sequences of the virus are present or not. In addition, the viral load, i.e. the concentration of the viral genetic material, can be determined.

CT value

The CT value is a term often used in association with PCR tests. CT stands for Cycle Threshold and indicates the actual viral load in a given sample. It refers to the number of cycles that a real-time PCR has run before fluorescence, i.e. virus detection, is measurable. A high CT value indicates a low viral load in the sample. However, it doesn't necessarily reflect the infectiousness of the persons concerned.

To evaluate the PCR test, a lot of different information is needed, such as:

  • the course of the disease
  • the occurrence and nature of the symptoms
  • possible contacts with infected persons
  • staying in risk zones
  • possible virus mutations

Further scientific data on PCR tests can be found on the following page: https://pubmed.ncbi.nlm.nih.gov/33595397/

When and by whom should the PCR test be carried out?

A PCR test should be carried out whenever you

  • show symptoms that could be related to a COVID-19 infection
  • have had contact with a COVID-19 infected person
  • are contacted as part of Contact Tracing or Large Scale Testing

The PCR test is carried out by qualified professionals in a designated facility. These include, for example, the various laboratories, the Large Scale Testing stations or hospitals. The National Health Laboratory (Laboratoire national de santé, LNS) also has the possibility of sequencing a sample to determine the type of coronavirus (mutation).

The analysis of a PCR test usually takes 24-48 hours. However, this may vary depending on the capacity of the laboratory and the number of tests performed daily. 

Antigen test

Rapid test by nose or mouth swab

These rapid tests have been available in Luxembourg since November 2020. The sensitivity of these tests is somewhat lower than that of PCR tests, as no amplification of the genetic material takes place in this procedure, but viral proteins are to be identified directly. Depending on the test and patient group, it is between 70 and 90%. These tests are mainly used to identify highly infectious persons.

The advantage of the rapid test over the PCR test is that a result is available within 15 minutes. Thus, in the case of a positive result, the affected persons can immediately go into self-isolation and the persons who were in close contact can proceed to self-quarantine. Unlike the PCR test, the antigen test looks for proteins of the pathogen.

There are several types of rapid antigen tests. Rapid tests performed by a nasal or throat swab can only be performed by trained personnel, while self-tests can also be conducted without specialists. In all cases, the test looks for viral proteins, which can be taken either from the mouth, the front or back of the nose, or even from saliva.

When and by whom should the rapid antigen test by nose or mouth swab be carried out?

The carrying out of this test (nasal or mouth swab) is limited to health care workers or specially trained personnel. In this case, a positive result should be considered as a suspicion of a COVID-19 infection and confirmed by a PCR test. In the case of a negative result, a high risk of infection can be ruled out at that time, but the result only serves as a momentary snapshot. Hygiene and protective measures must continue to be observed.

This test is intended for the rapid detection of an infected person in the presence of symptoms, but can also be used as a screening method, for example for visitors to a nursing home.

In the event of a positive test result, the person who carried out the test must report this to the Contact Tracing team using the form provided for this purpose.

Antigen self-test (nose swab)

So-called antigen self-tests have recently become available, which can be carried out by nasal swab without the person carrying out the test needing specific qualifications. These tests can therefore be carried out by whoever wishes to do so.

However, the accuracy can strongly vary. Depending on the test and the person, it is between 80 and 95% for symptomatic persons and between 50 and 60% for asymptomatic persons.

The accuracy of the test is optimal between the first and fourth day after the appearance of the symptoms, i.e. during the peak of the infection.

If the test result is negative, a PCR test should be carried out in the event of worsening or continued symptoms. In this case, contact your general practitioner.

However, if the individual does not show any symptoms and has not had any contact with an infected person, the negative test result can also be considered as such.

If the test result is positive, an infection must be assumed, especially in the case of symptoms. The person who tested positive must go into self-isolation and report the positive result online to the Health Directorate using this form. Afterwards, he/she must inform all persons with whom he/she had high-risk contact so that they go into self-quarantine.

All persons concerned will be contacted by the Health Directorate as soon as possible.

The result of the PCR test only confirms the initial positive finding of the antigen self-test. 

When and by whom should the rapid antigen self-test by nose swab be carried out?

This type of test can be carried out by any individual. It is intended to be used as a regular check, or to help in the rapid detection of a SARS-CoV-2 infection when symptoms occur. The positive result of an antigen self-test must be reported via this page.

Please note: Not every rapid test is approved as a self-test! A list of tests recommended as antigen self-tests in Luxembourg is available here.

An antigen self-test performed at school, sports or at work under the supervision of a person selected for this purpose must be reported via MyGuichet.

Important: Please refer to the provided package leaflet for detailed instructions before using the test. 

Spitting test (salivary test)

This type of test is a PCR test, which means it works on the same principle as a nose swab, with the difference that it is carried out using a saliva sample. Once the sample has been collected, it is analysed according to the PCR test procedure.

When and by whom is the spit test performed?

Only health care professionals or specially trained personnel can perform this test. Currently, this type of test is carried out as part of the Large Scale Testing, namely at the Howald test station.

Antibody test (serological test)

Antibodies are part of the immune response and help neutralise the virus in the body.

The antibody test, which is not to be confused with the antigen test, shows whether a person has antibodies to the coronavirus or not. This test indicates whether a person may have been infected with SARS-CoV-2 in the past, even without knowing it, or whether antibodies to the virus have been formed after a vaccination.

Rapid antibody test

Rapid antibody tests for non-experts are available on the market, but they currently show lower reliability than laboratory tests.

What is being tested?

A few droplets of blood from the finger are used to analyse whether antibodies against the spike protein of the coronavirus (SARS-CoV-2) are present.

What does a positive or negative test mean?

Positive test: The person has antibodies against the spike protein of the coronavirus, which means that either they have been in contact with the virus in the past or that the vaccine has caused an immune response. You could say that some protection has been built up. Scientifically, however, there is no limit above which one is considered protected. A high or low level of antibodies does not necessarily mean that you are immune, as antibodies are only a small part of the body's overall immune response.

Negative test: No antibodies against the spike protein are present at the time of testing. This can have different causes:

  • You have never been in contact with the virus and you have not yet been vaccinated
  • You have been vaccinated, but your body has no antibodies at the time of the test
    • The body has never developed antibodies and the vaccination has not triggered an immune response. You may be inadequately protected.
    • The body never developed antibodies but still responded well to the vaccination. Antibodies are only part of the overall immune response, and other parts of the immune system are quite active. Scientifically, this is considered proven, but such tests are not as easy to carry out as an antibody test.
    • The body has developed antibodies, but no longer produces antibodies at the time of the test due to lack of need. In this case, one can assume a certain protection because the body has built up its immune memory accordingly.

Why should I get tested?

You can get tested to find out if you have developed antibodies. In general, vaccination is very effective and there is nothing to worry about. However, if you are considered vulnerable, or if you are taking immunosuppressants, it can still be interesting to get tested, as these people can receive another dose of the vaccine to boost their immune response.

The Health Directorate uses these tests to monitor the general situation in the country in order to get an overview of:

  • how many people have been in contact with the virus or have been vaccinated
  • how many people have developed a certain level of immunity, in order to be able to possibly adapt the general sanitary measures

The testing and vaccination strategies are key elements in the fight against this pandemic. If you have the opportunity to do any of the above tests, please do so and contribute to reducing the spread of the virus.

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