Molecular swab tests, antigenic tests or rapid swab/saliva tests, and even blood serological tests. It may be difficult to navigate through this jungle of tests in order to detect a positive or negative SARS-CoV-2 result. However, what are the advantages and disadvantages of each type of test?
Let’s start from two fundamental concepts that form the basis of any test:
- Sensitivity: this means that the test detects the presence of COVID-19, even at low levels. How low depends upon the test and the threshold that the test manufacturer makes known, in order to market the test and receive the IVD (In Vitro Diagnostics) certification.
- Specificity: this means that the test detects only the virus you’re looking for; in this case, SARS-CoV-2 and no other similar viruses. The kit manufacturers also declare the specificity.
What are the main tests and what are their characteristics?
MOLECULAR SWAB TESTS
We wrote about this in an article back a little while ago — let’s summarize how they work, and what the most important features are.
For the purposes of clarity, the swab is only a tool for taking the patient’s nasopharyngeal sample. It’s a bit annoying, especially when the “taker” takes the sample from the throat. The effectiveness of the molecular swab depends very much on the operator’s ability to take the sample; the risk is to raise false negatives because not enough sample has been taken. The fact that there is not enough sample may also be due to a low viral load. Both of these events explain why we sometimes hear that the swab result swings from negative to positive. The sample is then analyzed with a technique, Real-time RT-PCR, which indicates the presence of genetic material of the coronavirus (i.e. its RNA).
But how can you be sure that it’s exactly the RNA of SARS-CoV-2 and not another coronavirus that causes the cold? The specificity of the molecular test (i.e. the fact that it’s been “drawn” on the specific gene sequence of SARS-CoV-2) tells us.
The advantage of the molecular test is that it’s very specific, sensitive and is diagnostic. The disadvantage is that it requires a sampling that must be performed by specialized personnel, a specially equipped laboratory, and requires at least two hours to provide a result. Unfortunately, due to current workloads and the lack of reagents, results can now take as long as a few days.
The molecular test can also be performed on a saliva sample, as announced by Dr. Capobianchi, Director of the UOC Laboratory of Virology, Lazzaro Spallanzani National Institute for Infectious Diseases in Rome, Italy. However, although the first data are encouraging, there’s not yet a validated and certified test on the market that would be a valid alternative to a molecular buffer, as it would be less invasive and less operator-dependent. The limits are the presence of inhibitors in the saliva that make the test less sensitive and less practical to automate the processing of samples.
These tests detect the presence of viral proteins, called antigens. The collection is carried out just as with molecular swabs on a nasopharyngeal sample, and then we use a “rapid” detection system, which exploits the binding between specific antibodies and antigens present in the specific sample being taken.
The advantage of antigenic tests lies in the speed of their execution (about 15 minutes). In fact, these tests are used, for example, in airports to quickly test a large number of passengers. The limits are dependent upon the specificity and sensitivity, which are also influenced by the time between the collection and execution of the test. The more time elapses, the less reliable the test is.
It is, however, a valid alternative to molecular swabs in the case of screening (i.e. the execution of tests on a large number of people simultaneously where a small margin of error can be tolerated). However, if a person is positive for the antigenic test, he or she must be swabbed for confirmation.
The antigenic test could also be very useful in monitoring, for example, students and school staff, who could all be tested each week, in order to prevent (further) outbreaks.
These tests are based on the detection of antibodies generated by our body, once we’ve come into contact with a virus. To do this, it’s therefore necessary that the antibodies present in the blood bind to the specific antigens of the coronavirus. In addition, in this case, both the specificity and sensitivity of the test are essential.
There are two types of serological tests:
- Rapid or “Pricked” Tests: we described these in a recent article (Italian-language only), and which are qualitative (i.e. they don’t indicate how many antibodies you’ve developed but only whether you’ve developed IgM and/or IgG). Remember that IgM are the antibody immunoglobulins that develop a few days after the onset of the infection, while IgG are the immunoglobulins that prove whether you’ve come into contact with SARS-CoV-2 and provide you with some immunity for a specific period of time.
Rapid tests are inexpensive, easy to use and yield a result within a few minutes in a similar way to pregnancy tests (i.e. indicating a band for each of the Immunoglobulins). In a previous article (in Italian only), there’s also an Italian-language video in which we perform the test from a drop of blood.
- Conventional Serological Tests: these are based on a blood sample and are a quantitative test; that is, that it tells you how many antibodies are circulating within your body and uses a technique called ELISA (Enzyme-Linked Immunosorbent Assay). This test must be performed by specialized laboratories and it yields a result in just a few hours.
Both allow for the measurement of antibodies that the immune system produces in response to SARS-CoV-2 infection. Such an investigation, therefore, does not provide certainty that the patient has an infection “in progress”; it could be contagious, in the case of detection of IgM (which are usually present with symptoms), while in the case of detection of IgG, the patient is very likely no longer contagious.
Serological tests are therefore not intended to diagnose COVID-19 and don’t provide definitive information about an individual’s contagiousness.
In fact, antibodies occur about one week after infection (i.e. the presence of SARS-CoV-2 in the body); presence of the virus that can be detected with the molecular buffer.
In conclusion, serological tests are useful in carrying out screening and in understanding the state of spread of the virus throughout the population (i.e. the seroprevalence, and this is where the Italian study began): the COVID-19 Serology Project (click here for more information).
Watch the (Italian-language) video in which our colleague, Dr. Federico Sebastiani, performs the rapid test from a drop of blood, and arrives at the result within just a few minutes.
This post is also available in: Italiano