How is the COVID-19 diagnostic test performed? Let’s find out together.
The diagnostic test for the coronavirus is a laboratory test that is performed using body secretions to determine whether or not someone is infected with the virus.
Which secretions can be used?
The swab used to collect the secretions can be rubbed on the pharynx (i.e. “in the throat”) or lower, in the trachea. A bronchoalveolar lavage can also be done in the bronchi. The swab can be applied in any of these areas because these are where the virus lurks: first it replicates in the upper part of the airways, then gradually moves down, finally reaching the lungs where it occasionally produces a major infection/inflammation, called pneumonia.
The swab: What to expect
The swab can be performed at home, at the hospital, or remotely wherever there is someone suspected of having the virus. The sample is taken with a cotton swab which is then placed in a sterile test tube and transported to the best equipped hospital. A normal microbiology lab is not equipped for this type of investigation. The swab tests are carried out in biosafety level 2 (BSL-2) laboratories, which are limited in number.
Why do only a few specialized laboratories test for the COVID-19?
The COVID-19 test is only carried out by a few specialized laboratories because resources are limited. Many specialized tests, such as the one for the Coronavirus, require specific mixes to be carried out.
Is there only one test?
First of all, it must be said that there is no standard test worldwide to diagnose COVID-19. As of today, there are six protocols available developed by the Chinese, German, Japanese, Thai, and American institutes. More information can be found on the World Health Organization website (1).
How does the test work?
Each of the six protocols works like a detective: it looks for a specific print of the virus. Just as a detective looks for different fingerprints, each protocol looks for different prints of the virus inside a swab sample of a patient.
In the following image you can see the “fingerprint” of genetic material, which the protocol developed in Berlin looks for.
How does the protocol work? (If you are already getting bored, skip this step and move on to the next topic!)
For those who want to know more, here is a simple explanation:
- At the right temperature, the double-stranded RNA of the virus opens up.
- Ready-made pieces of RNA created in laboratories are added to the mix.
- The RNA polymerase, for gene writing/amplification of the pieces of the viral patterns, is activated.
The genetic material is amplified through various reactions until millions and millions of copies are created.
If enough viral RNA exists in the sample, the test will detect it.
The following video explains the PCR system that is used to perform this type of testing: https://www.youtube.com/watch?v=QPOMrRUOMaY
Certain checks have been carried out to ensure the test performed does not produce significant “errors”, such as detecting other viruses or coronaviruses other than the SARS-CoV-2 responsible for the COVID-19.
Below is a list of the results obtained after using the test; checking that it does not cross-link with other coronaviruses.
Below is a list of other known respiratory viruses, all of which tested negative.
How reliable are these diagnostic tests? There are two types of reliability. One related to the test itself, the other related to the sample that the test must consider.
In some cases the test can be “wrong”, i.e. it can say that there is a coronavirus when there is none, or that there is no coronavirus even if it is actually present in the sample. This type of error exists and it is taken into account, however it is quite rare.
The test can be carried out on a sample that is not reliable for many reasons. For example, the sample taken with the swab may have insufficient virus (low “viral load”), or it might have been taken at a time when the patient did not have a substantial amount of the virus. This topic will be further explained in the next article.
Marco De Nardin, M.D., Anesthesiologist, Critical Care Doctor
- (1) https://www.who.int/emergencies/diseases/novel-coronavirus-2019/technical-guidance/laboratory-guidance
- (2) https://www.who.int/docs/default-source/coronaviruse/protocol-v2-1.pdf?sfvrsn=a9ef618c_2