Salivary tests to detect the coronavirus are not among those allowed to receive the green pass and the sensitivity was evaluated between 53 and 73%. Abrignani: They cannot in any way replace the vaccine
Sui salivary tests There has been a heated debate for months to detect the presence of Sars-CoV-2. The issue returned to the fore after the president of Veneto Luca Zaia, interviewed by Courier service
, he said that in order to guarantee the school in the presence he intends to carry out sweep screening with these tools. The plan developed together with the University of Padua provides for sentinel schools, or, explains Zaia, a network of dozens of institutes scattered throughout each province where the level of circulation of Covid is constantly monitored, making swabs after swabs. Salivaries to find the asymptomatic and then, in case of positive, simple nasal swabs for the whole school. The goal in Veneto, the president clarified, is to vaccinate as much as possible in view of D-Day which will be the opening of schools. Strongly in favor of salivary swabs in schools also Rossano Sasso, undersecretary of the Ministry of Education: Immunization alone is not enough to avoid the hypothesis of quarantines and school closures in the face of the positivity of students, teachers or staff of the institutes. Salivary swabs can be a valid support for monitoring the situation of school communities and avoiding the use of distance learning: I ask President Draghi and Minister Speranza to work towards the massive adoption of these devices.
Are salivary tests used in Italy today? Are they valid for the green pass?
Yes, in Italy they are used, but – be careful – they are not among those allowed to receive the green pass, as well as self-tests and serological tests. A circular from the Ministry of Health dated May 14 clarified under what circumstances they can be used: The saliva sample can be considered an option for the detection of infection, preferably within the first five days of the onset of symptoms, if not it is possible to obtain gold / nasopharyngeal swabs. In addition, it says, salivary tests can be a useful tool for monitoring and controlling infection in the school setting. Some studies published in 2020 found sensitivities between 53 and 73%.
What are the limitations of these tests?
The sensitivity not very high and decreases after the first five days onset of symptoms. Furthermore – explains the Ministry – the correct collection of the salivary sample is a crucial step. Saliva samples can be heterogeneous (oral saliva, posterior oropharyngeal saliva) and different techniques and collection sites can have an impact on the sensitivity of the method. Additionally, saliva samples can be mucous and viscous, resulting in processing difficulties with existing automated RNA extraction or extraction / amplification methods and equipment. Salivary tests have an indication for situations in which in a short time, in a few minutes, you have to allow a large number of people to enter the classroom, by plane or train – explains Sergio Abrignani, full professor of General Pathology at the University State University of Milan and director of the Romeo and Enrica Invernizzi National Institute of Molecular Genetics, as well as a member of the Technical-Scientific Committee, guest of Agor Estate on Rai Tre -. They have a very limited sensitivity but just know it, they mitigate the risk but there is nothing that eliminates it. You get the vaccine up to three times and you are protected from death in 98% of cases, from severe disease in 95% and from infection in 80%. The salivary swab occasionally makes sense for screening, but not for the everyday. To say that the swab or salivary test replaces the wrong vaccine cannot be an alternative.
Even in recent months, the reopening of schools was linked to the hypothesis of carrying out carpet tests on students and there was talk of rapid salivary swabs. What are they?
Rapid antigen tests, less precise than molecular ones, detect the presence of the virus not through the nucleic acid (Rna), but through its proteins (antigens): they provide results in less than half an hour and can be performed anywhere. Damage for a certain percentage of fakes: the sensitivity (probability that a sick subject is positive to the test) varies between 70 and 86% and the specificity (probability that a healthy subject is negative to the test) between 95 and 97 for hundred. If the rapid test is positive, confirmation with the molecular swab may be required. Rapid salivary tests are performed on a saliva sample, so a nasal-or-pharyngeal swab is not necessary: this makes them particularly suitable for children.
Have rapid salivaries been studied in Italy?
A first trial was carried out at the Spallanzani Hospital in Rome, last October. The samples, collected in the schools, were sent to the laboratory of the San Camillo Hospital which gave the result in the evening. Only in the case of a positive coronavirus test was the sample sent to Spallanzani for the subsequent molecular test. The result of the experiment was not as expected. The rapid salivary tests are not sufficiently efficient as they have a sensitivity of less than 20%, explained the Spallanzani experts. Basically, the salivary swabs made on the spot proved unreliable and required confirmation in the laboratory with the molecular test, thus canceling the effect of an immediate response. The possibility of using molecular salivary tests (therefore not rapid) is also being studied. A study by the University Hospital of Padua, coordinated by Mario Plebani, showed the high efficacy of molecular salivary tests: 98% of positive cases were confirmed by the classic swab.
What diagnostic tests are used in Italy today?
They are of three types: molecular, antigenic and serological. The molecular swab detects the genome (Rna) of the virus in the biological sample: the first choice, for example in case of symptomatic suspicion, close contact of confirmed case manifesting symptoms, in healthcare worker screenings, in subjects in contact with frail people or for entry into closed communities. Rapid antigen tests offer the advantages of speed and low cost, despite the limitations mentioned above. There are also non-rapid antigenic tests, to be performed in the laboratory: they are sensitive to some proteins of the virus and not to its Rna. Finally, serological tests show whether there is (or has been) an exposure to Sars-CoV-2, but are unable to confirm whether the infection is taking place. For this, positivity requires a control molecular test.
What are the limitations of rapid antigens?
They can be negative if the antigen concentration is below the detection limit of the test (for example, if the sample was taken too soon after the hypothetical time of exposure to the virus) or if the sample was improperly collected, transported or stored. Therefore, a negative test result does not rule out the possibility of infection and, if there is a suspicion of Covid, it should be confirmed by laboratory tests. Moleculars seem to have a greater sensitivity before the onset of symptoms, while in the phase immediately following the onset of infection they have shown an efficacy similar to that of rapid antigenic tests.
August 28, 2021 (change August 28, 2021 | 16:18)
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