Now that Covid-19 has been in our daily lives for more than a year, more and more tests and rapid tests for Covid-19 are available on the market. Not only is the Dutch government pushing for Covid-19 testing, but many airlines also require you to show a Covid-19 travel certificate if you want to travel. But which test should one choose? What is the difference between PCR tests, rapid antigen tests, antibody tests and PoC tests? We explain it all here.

When talking about Covid tests, you have probably heard of PCR tests. PCR tests are the best known and most commonly used Covid tests. The letters PCR stand for polymerase chain reaction. This means that the test converts the RNA of the virus into DNA and copies this DNA millions of times, after which a molecular test can detect the DNA. The PCR test can detect the RNA from about 2 days before the onset of symptoms to sometimes 2 months after the onset of symptoms.

How does it work exactly?

A SARS-CoV-2 PCR test consists of several steps.

1. Material is taken from the nose and throat using a swab.
2. In an isolation step, the RNA is purified from the collected material.
3. The purified RNA is then converted into DNA using a protein called reverse transcriptase (RT).
4. Amplification step: Primers are added to the converted DNA.
5. The primers label and amplify a unique part of SARS-CoV-2.

The multiplication is carried out in cycles. In each cycle, the number of copies of the unique part is doubled, resulting in a 1 on 1 copy. The amplification step usually consists of about 40 cycles. This sometimes produces billions of particles of the unique part of the DNA. The PCR machine detects when this number of unique particles exceeds a certain threshold and the test is then positive.

Common PCR machines indicate how many cycles it takes to become positive. The number of cycles is the Ct value. If the PCR comes out positive with a low Ct value, it means that there was a lot of RNA of the virus in the material originally collected. A high Ct value means that there was little RNA in the initial material.

How long does a PCR test take in a laboratory?

The duration of the PCR test varies between 45 minutes and 4 hours. This depends on the equipment used.

Advantages of the PCR test

One of the biggest advantages of a PCR test is that this test can detect the virus even if there is only a little RNA of the virus on your stick. This way you can test better and more specifically. Most PCR tests are 100% accurate.

Disadvantages of the PCR test

The major disadvantage of a PCR test is that these tests can only be done by people who are trained to do them. Also, it can take several hours before you get a result.

Even one to two months after contracting the virus, the result of a PCR test is often positive. This can be a problem for travellers. Many countries now offer opportunities for travel. The traveller should inform himself well about the rules.

While the PCR test and the rapid antigen test use nasal mucus or a sample from the throat, an antibody test looks for antibodies against covid-19 in your blood. The antibody test, also called a serological test, shows if there are specific antibodies against SARS-CoV-2 in your blood. This test shows whether you have had the virus.

A drop of blood is tested for Covid-19 antibodies. These antibodies are made 7 to 21 days after infection. This test does not check if you are currently sick, but it does check if you have been infected with Covid-19 in the past.

How does an antibody test work?

It can take 1 to 3 weeks for your body to make the right antibodies and for them to be present enough in your blood so that they can be measured. For a serological test, some blood is taken. The blood is then analysed in a laboratory.

Depending on the period after infection, IgA, IgG and IgM antibodies can be measured. The first antibodies that the body produces are of the IgM type. IgM can be used to show that an active infection is taking place. This process only starts after 2-3 weeks.

Over time, the amount of IgM antibodies decreases and so-called memory cells, IgM, are produced. These ensure that when the body comes into contact with the foreign organism again, a large amount of antibodies is quickly produced. IgM indicates that someone has come into contact with the virus in the past. There are also IgA antibodies. These are formed temporarily between the IgM and IgG phases.

Advantages of the antibody test

The advantage of the antibody test is that you know after the test whether you have already had Covid-19.

Disadvantages of the antibody test

An antibody test can only be used to find out if you have had Covid-19 in the past. This test is not suitable for finding out if you currently have Covid-19.

Watch out!

There are also rapid antibody tests. These are usually not very accurate. Please check well with your test provider.

A PoC test, PoC is the abbreviation of Point Of Care, is a test that can give a result within minutes. This type of test has been around for a long time and is mainly used in hospitals to perform laboratory tests by specialised laboratory staff.

PoC tests have also been developed for the coronavirus outbreak. These tests can be based on different types of SARS-CoV-2 diagnostics, including the antigen test, which looks for proteins of the virus, or the PCR test, which converts the RNA of the virus into DNA and then amplifies it.

PoC tests are based on a variety of technologies and formats, both old and new: conventional antigen-based assays to detect viral proteins; isothermal nucleic acid amplification and loop-mediated isothermal amplification (LAMP) to detect viral genetic material; and newer CRISPR-based diagnostic assays that detect viral genetic material but in fewer steps and without polymerase chain reaction (PCR) machines.

PoC tests are easy to use in high-risk environments, such as care or educational settings. PoC tests are a useful and rapid addition to laboratory-based PCR diagnostic tests.

Advantage of PoC tests

PoC tests can provide reliable results within minutes. This is very important in hospitals, for example, where patients need to be tested quickly in order to send them to the right department.

Disadvantage of PoC tests

Although PoC tests can revolutionise testing for covid, it is true that the test can only be performed by trained personnel. Moreover, the reliability of the test does not depend on the testing method but on the personnel who perform the test. In the Netherlands, this type of diagnostic test must also involve a laboratory with a certificate of proficiency and a molecular biologist or a doctor-microbiologist.