Raise your hand if you haven’t used them at least once. After the first waves of Covid-19, the use of quick do-it-yourself tests for the diagnosis of Sars-CoV-2 infection has spread like wildfire also in our country. All the more so in a relatively safe phase such as those in Italy today: with a limited number of cases, also a consequence of the high level of participation in the vaccination campaign. That said, some limitations remain. And the almost absolute circulation of the Omicron variant, in its various declinations, also seems to emerge in a more significant way.
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Rapid and molecular tests: results not always identical
This is the data that emerges from a study published in the British Medical Journal. It was led by a group of Dutch researchers, who between 21 December and 10 February last involved almost 6,500 adults who went to three centers of the public health service (in Breda, Roosendal and Tilburg) with symptoms compatible with those of the SARS infection. CoV-2. All underwent a molecular oropharyngeal swab before being asked to repeat a (quick) test at home over the next three hours and then fill out an online questionnaire.
Different results in the hospital and at home
The two investigations did not always give the same result. Starting from the diagnoses obtained with molecular swabs, the researchers found that the sensitivity of the three types of rapid swabs used (among the most popular in the Netherlands) was between 70 and 81 percent. Down by ten points – on average – compared to the values found before the arrival on the scene of the Omicron variant. These are the values obtained following a nasal sampling and in 2 cases out of 3 results below the standards indicated by the World Health Organization: corresponding respectively to 80 (sensitivity, the ability to recognize positives) and 97 percent (specificity , i.e. the ability to correctly identify a negative sample).
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Sensitivity increases with oropharyngeal sampling
The situation improved after including a throat sample in the sample. However, no rapid swab, as recalled in an editorial appeared in the same magazine, “has reached levels of accuracy comparable to those advertised by the producers”. According to the researchers, coordinated by epidemiologists Carl Moons and Janneke van de Wijgert (University of Utrecht), “the nasal one, alone, is not always sufficient to detect Omicron variant infection”, whose prevalence in the Netherlands during the study period was estimated to be close to 90 percent.
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“Manufacturers should consider including oropharyngeal sampling in their indications for use of rapid antigen tests,” they conclude. Indication that, in many of the products in use in Italy, is actually already provided. But that is difficult to apply when these tests are used at home.
“The double sampling, starting from the throat and then passing through the nose, is more effective in intercepting a greater number of viral particles – he specifies Giancarlo Icardi, director of the Hygiene laboratory of the San Martino hospital in Genoa, as well as a member of the working group on vaccines of the Italian Society of Hygiene (SItI) – and is more indicated in a phase in which the prevailing variant tends to populate the upper respiratory tract. But it is difficult for anyone to decide to do it on himself. To this we must add that, beyond the single or double sampling, we are talking about a procedure conditioned by a high intraoperative variability “.
The results, in essence, may change depending on the operator carrying out the test. A gap that tends to widen when the comparison is made between a professional and the same (potential) patient.
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Less sensitive rapid tests also due to Omicron’s “fault”?
At the moment it is only a hypothesis. The different sub-variants of Omicron, with the latest evolutions of the Spike protein, may have made even tests less precise, the quality of which has however grown in tandem with the pandemic. And it was confirmed by the Ministry of Health, which a year and a half ago began to equate third generation ones with molecular swabs. Their use has helped to ease the pressure on the health service in the most critical moments of the pandemic, characterized by the systematic exceeding of the threshold of one hundred thousand new positives every day. And it has remained constant over time, thanks also to the growing number of people vaccinated or previously infected: as such, they are almost always at lower risk in the event of a new infection. Over time, however, the opposite excess has also been reached, with thousands of unrecorded positives. And free, therefore, to move.
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Covid is less scary, but the positives isolate themselves
Hence, also in light of the latest evidence, the need to reiterate some recommendations: “At the moment the pandemic in Italy is under control and Covid-19 is less scary – admits Icardi – this climate of greater security has favored a massive increase self-test procedures. However, it should be remembered that, even in this case, any positivity must be followed by a period of isolation to protect others. , however, is to contact expert personnel for the execution of the test. Today, doctors, pharmacists and the staff of the analysis laboratories are on average experienced and able to evaluate even the most appropriate time to carry out a swab. an additional annoyance, but which allows us to feel more confident: whatever the test result “.