The request for anti Covid tests flew due to the leap of infections and “preventive” checks due to the holiday period. While the molecular remains the safest option on a diagnostic level but also the most difficult to find given the extraordinary demand, and of course the most expensive, there are several options. Of these, the focus is now on the third generation rapid tampons, which although more reliable and sensitive, are however not a valid alternative to the molecular.
The evolution of “rapid”
The rapid antigenic tests detect the presence of viral proteins (antigens) and there are several types available: the first generation ones are immunochromatographic, the second ones are immunofluorescent. The latest generation tests, always carried out using a nasopharyngeal swab, differ in the analytical method: that is, immunofluorescence with microfluidic reading. They are typically available in healthcare facilities or testing laboratories and take longer. “Those of the third generation are characterized by high sensitivity and specificity but in any case they cannot be superimposed on the molecular swab, which remains the reference exam for the research of SARS-CoV-2”, explains Carlo Federico Perno, director of the Hospital’s Microbiology pediatric Infant Jesus.
Omicron variant and sensitivity
Also for the third generation rapid tampons, “a positive result generally offers a reliable answer, while a negative one does not give a guarantee of real negativity”. Yes, therefore, for the expert to use “for the diagnosis of positivity, it is better to avoid using them to diagnose the end of the disease”. Among the reasons for concern is the suspected lower sensitivity to the Omicron variant by rapid antigen tests done in pharmacies. Almost one in two is in fact a false negative, according to what was stated by Guido Rasi, consultant to the commissioner for the Covid emergency Francesco Paolo Figliuolo and scientific director of Consulcesi. «With the Omicron variant set to become predominant, rapid antigenic swabs risk becoming useless. The new version of the SARS-CoV-2 virus seems to be able to escape the most frequently used diagnostic tests today ». And he adds: “Now more than ever it is essential that health professionals continue to keep up to date on the evolution of the virus and on knowledge in both the diagnostic and therapeutic fields”. The Food and Drug Administration (FDA) has also emphasized that the rapid antigenic swabs are not very sensitive and therefore unreliable in detecting positivity to the Omicron variant. “Preliminary data indicate that about 40% of people positive for the omicron variant may be negative on rapid tests, almost 1 in 2”, explains Rasi, according to which the “loss” of reliability of rapid antigen tests could make it “more complicated and difficult” contain the pandemic.
The limits of “do it yourself”
However, a problem consists in how the swabs are performed, that is the accuracy of the sampling. This is why the expert warns against “do it yourself” tests. «Self-tests, done at home and perhaps badly, by non-professionals, represent a risk. A negative result in this case is more probable and represents a false certainty ». Relying on local health facilities is therefore the preferable option. «In the pharmacy and – explains Roberto Tobia, national secretary of Federfarma – there are no shortages of tampons, if not those related in some situation to the Christmas holidays which, due to the increase in demand, have recorded some slowdown in supplies. But now he’s back. ‘
Molecular in “minipool”
For public health, the greatest danger is that false negatives escape. One possibility could be to perform molecular tests in “minipool”, proposed by Carlo Roccio, biologist and president of Clonit, a biotech company founded in Milan in 1987, with widespread sampling through the same network of pharmacies but then providing for the examination real by authorized laboratories. “Consider a collection of 100 samples, with the minipools they would become 20 analyzes in the first step, if the positives with the current prevalences were 2 and then, secondly, another 10 samples would have to be analyzed individually, the analysis of 30 samples would be reached out of 100 total, with a 70 per cent saving on the cost of both extraction / purification and gene amplification reagents ». According to Roccio, this procedure “supported by the very high sensitivity of the molecular method, in our opinion, could be applied in schools, RSAs, the prison population and companies with a significant and effective reduction in the costs of logistics and actual diagnostics”.