Nasopharyngeal swabs, salivary swabs or serological tests? Here is the classification of the Spallanzani Institute
Test, test, test. This is the watchword at the time of Covid-19: every single test can be useful to identify a sick person, reveal an asymptomatic and break the chain of contagion.
But there are tests and tests. To classify them (and judge them) is the Spallanzani Institute of Rome.
THE NOSE SWABS
Nasopharyngeal swabs have always been considered the most reliable test, but in days of health emergency they were scarce and were administered with a dropper.
Now the availabilities are greater. And there are different methods of examination: it can be molecular (identifies the viral genes most expressed during infection) or antigenic (identifies viral proteins).
In phase two, serological tests proved important to test employees of companies returning to work after the lockdown. The classic one detects the presence in the blood of specific antibodies that the immune system produces in response to the infection, their type (IgG, IgM, IgA), and possibly their quantity.
Rapid serological tests are based on the same principle as the classic ones, but they are simplified and give only qualitative answers, that is, they say only if specific antibodies for the virus are present in the organism.
THE SALIVARY TEST
Now, there are those who hope in salivary swabs, to be able to carry out fast and mass screening in schools, but the Spallanzani Institute slows down: either they require laboratory equipment or they are not very reliable.
Below is the report of the Spallanzani Institute.
1. Molecular test
It is the so-called "swab", the currently most reliable test, the one that is used today for the diagnosis of coronavirus infection. A sample of the patient's respiratory tract is taken with a long stick similar to a cotton swab, preferably from the nasopharyngeal mucosa or, where possible (in the hospital), sputum or bronchial wash or bronchial aspiration is used. This sample is then analyzed through molecular methods of real-time RT-PCR (Reverse Transcription-Polymerase Chain Reaction) for the amplification of the viral genes most expressed during the infection. The analysis can only be carried out in highly specialized laboratories formally identified by the health authorities, and takes on average two to six hours from the moment the sample is sent for processing in the laboratory.
2. Antigen test
This type of test is based on the search, in the patient's respiratory samples, for viral proteins (antigens). The sample collection methods are completely similar to those of molecular tests (nasopharyngeal swab), the response times are very short (about 15 minutes), but the sensitivity and specificity of this test are lower than those of the molecular test. , therefore the positive diagnoses obtained with this test must be confirmed by a second molecular swab. This "quick buffer" has recently been introduced for situations, for example in the screening of passengers at airports, where it is important to have a quick response. Antigenic tests exist in the POCT version (ie they can be performed at the sampling site), and in the “laboratory” version (ie they require laboratory equipment).
3. Classic serological test
While the "swab", be it molecular or rapid, detects the virus or its proteins in the patient's respiratory secretions, the serological (or immunological) test detects the presence in the blood of specific antibodies that the immune system produces in response to the infection , their type (IgG, IgM, IgA), and possibly their quantity. This test, also called ELISA or CLIA or IFA depending on the technology used, requires a venous blood sample, and is carried out in specialized laboratories.
4. Rapid serological test
The rapid serological tests are based on the same principle as the classic ones, but they are simplified and give only qualitative answers, that is, they say only if the organisms are present.
virus specific antibodies. These tests are easy to perform, have an average response time of about 15 minutes and can also be carried out outside the laboratory: a drop of blood obtained with a lancing device is deposited on the detection device, and the presence of antibodies is displayed by means of the appearance on the device of a colored band or a fluorescent signal read with a portable instrument. However, the reliability of this type of test varies widely, and the World Health Organization does not currently recommend the use of this type of test for antibody detection in patient care.
It is important to underline, in conclusion, that serological tests, by their very nature, are not able to tell whether the patient has an active infection, but only whether or not he has come into contact with the virus. When we become infected, our body takes a few days to produce specific antibodies, which then remain in circulation for a long time after the infection has been overcome and the virus is eliminated. A positive serological test person may therefore no longer have an infection in progress, and vice versa a negative serological test result does not exclude that the person is positive for the virus because
she has recently become infected. The classic serological test therefore has no diagnostic value, except in certain cases, in which the evaluation of the symptoms and clinical course of the individual patient must be associated; the search for specific antibodies carried out with serological tests is instead extremely useful in epidemiological studies, for the evaluation of the real circulation of a virus in the population after an epidemic event.
In conclusion, serological tests cannot be used for the diagnosis of infection in progress: for this purpose it is necessary to use diagnostic methods that detect the direct presence of the virus genome or its proteins Molecular test is considered the gold standard for diagnosis , while the other methods should be used if the possibility of accessing the classical molecular test is not available, or for purposes other than the diagnostic context, such as epidemiological surveillance.
5. Salivary tests
Tests have recently been proposed on the market that use saliva as a sample to analyze. Saliva collection is simpler and less invasive than nasopharyngeal swab or blood sampling, so this type of test could be useful for screening large numbers of people. As for swabs, also for salivary tests there are molecular type tests (which detect the presence of the virus RNA in the sample) and antigenic type (which detect viral proteins in the sample).
- The molecular salivary test tested at the Spallanzani virology laboratory uses one of the systems currently used for nasopharyngeal swabs, which has just obtained the CE mark for use also with saliva. However, this equipment does not have a high throughput: in fact it is able to carry out only 8 samples at a time, with sample analysis times of about one hour. Therefore its use is indicated only in special cases, such as the urgent confirmation of positivity found in the course of antigenic tests. Saliva generally does not lend itself well to use with highly automated laboratory equipment that is used to process high volumes of molecular samples, because it has variable density and would create problems for highly automated suction systems.
- Regarding antigenic salivary tests, our laboratory tested two solutions. The first showed sensitivity levels similar to those of rapid antigenic swabs, but the test must be performed in the laboratory, therefore, unless laboratory units are activated at the points where the collection is carried out, it is not usable in contexts of rapid screening (eg airports) since between sample collection, transport and acceptance in the laboratory, execution of the test and reporting, the results, although faster to be obtained than the molecular test, are not immediate. The second solution, on the other hand, is visually readable (so-called "soap"), does not require laboratory equipment, can therefore be used outside the laboratories and gives results in a few minutes, but applied to saliva (different context from that for which it is certified, i.e. buffer), at the first tests carried out it would appear to be less performing than the standard molecular test.
- serological tests cannot be used for the diagnosis of infection in progress: for this purpose it is necessary to use diagnostic methods that detect the direct presence of the virus genome or its proteins (antigens);
- Rapid antigenic tests on nasopharyngeal swabs can be useful in certain contexts, such as contact tracing of positive cases or rapid screening of numerous people;
- At the moment, antigenic and molecular tests on saliva samples are unlikely to lend themselves to the rapid screening of many people as they require an equipped laboratory;
- Molecular testing remains the gold standard for diagnosis to this day; the other methods must be used if the possibility of accessing the classical molecular test is not available, or for purposes other than the diagnostic context, such as epidemiological surveillance.
This is a machine translation from Italian language of a post published on Start Magazine at the URL https://www.startmag.it/innovazione/covid-19-ecco-i-diversi-tipi-di-test-e-tamponi-report-spallanzani/ on Tue, 29 Sep 2020 04:53:40 +0000.