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 works according to the same principle as the nose swabs.

If they are on the list of self-tests, these tests can be carried out without problem. 

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.

When and by whom should an antibody test be carried out?

An antibody test is carried out through blood test (a serum tube) in a designated laboratory or by taking a drop of blood from the finger (DBS - Dry Blood Spot). You can also take a blood test at any time at your own expense.

This test is useful, for example, if you suspect that you have been in prolonged contact with a person who has tested positive, or if you want to know the level of antibodies after a COVID-19 illness. 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.

As part of the Large Scale Testing, a representative group of the Luxembourg population, based on place of residence, age and household, is invited each week to provide a continuous view of the evolution of the pandemic. 

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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