The surge in tests for the diagnosis of SarsCoV2 virus infection has not led to an increase in positive cases and one wonders if this is due to the fact that the circulation of the virus is actually poor or if the cause is to be found in the characteristics of tests.
"The data collected from 2 October to today indicate that there is no downward trend, but that the situation of the Covid epidemic in Italy is constant", notes the virologist Francesco Broccolo, of the University of Milan Bicocca.
To indicate that there has not been an effective reduction in cases is the photograph of the data relating to Saturday 9 October and Saturday 16 October: it indicates an increase of 27% in the total tests, between molecular and rapid antigenic, which went from 344,969 to 472,535, and at the same time the percentage of positivity, calculated on the total of the tests, was reduced from 0.7% to 0.6%. "It is a slight decrease - notes the virologist - which must be carefully analyzed, as it is substantially due to an increase in antigenic buffers".
In fact, analyzing the data in detail, of the approximately 345,000 tests performed on October 9, the rapid antigenic tests were 240,995, i.e. 70% of the total, and the ratio between the total number of cases detected and that of antigenic tests alone gave as a result a positive index of 0.1%; on the same day, a positive rate of 2.4% was deduced from the 104,641 molecular swabs administered.
One week later, the molecular test situation was comparable to the previous week, with 114,721 swabs and a positive rate of 2.34%, while rapid antigen tests had seen a further increase, from 70% to 75%. of the total tests, with a positive rate of 0.08%.
The data of 2 October substantially confirm the trend, with a total number of swabs equal to 355,896, between molecular and rapid antigenic, from which an overall positivity rate of 0.9% emerged; going into detail, a positive rate of 0.08% emerged from the antigenic swabs alone, against 2.77% of that resulting from the molecular tests. "Consequently, the positivity rate of 0.6% and 0.7 - observes Broccolo - is a percentage that derives from a weighted average of molecular and antigenic tests", that is an average that is affected by the large number of antigenic tests quickly administered.
Another important element, continues Broccolo, is the fact that the tests that have the lowest sensitivity record the greatest increase: "rapid antigenic agents can only detect viral loads exceeding one million" copies of genetic material of the virus. present in a milliliter of biological material taken with the swab. "Molecular tests, on the other hand, are able to identify viral loads ranging from 10 million copies up to one hundred copies". This different sensitivity depends on the nature of the tests: the molecular ones analyze parts of the genetic material of the virus after having amplified them with the technique of the polymerase chain reaction (Pcr); rapid antigens look for the protein components of the virus called antigens.
Finally, Broccolo points out, it must be considered that the large increase in antigen tests is not evenly distributed in the population, but most likely it is mostly representative of adults of working age, who need the Green pass for their activity.