With a high incidence, group tests are not useful


Below 5% or 3%, pool testing becomes cost-effective

Several health professionals perform antigen tests on students at the Instituto de Educacin Secundaria Vivente Medina in Archena, (Murcia).MARCIAL GUILLNEFE
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In full return to school, with micron multiplying, health centers and hospital emergencies saturated, the concept of ‘pool testing’ or group tests is rescued as a possible formula to test the population and detect positives in a massive and at the same time profitable way. Here are some questions that will bring you a little closer to the reality of this test whose philosophy is to do more with less to save resources.

What is ‘pool testing’?

It involves forming groups of 10, 15, 20 people or more, from which samples are taken and mixed together to perform a PCR. If it is negative, it is understood that the entire group is free of Covid-19. If, on the contrary, “it were positive, the next step would be to do the test individually,” explains Rafael Cantn Moreno, head of the Microbiology Service at the Ramn y Cajal University Hospital in Madrid.

What are the advantages of these group tests?

First, it saves resources. Less tests are used, less plastic material, tubing, etc. It is simple, in a single PCR several people are tested. Fundamentally, it saves time and money. A fact: with a prevalence of 0.5% of the infection in a given city, group tests allow examining seven million people in less than two months.

Is it useful in any circumstance?

In Canton’s words, it is “a very interesting strategy in situations where the proportion of positivity is low.” However, with the positivity we have now, around 40% in primary school, pool testing “is not cost-effective. […] It begins to be so in an epidemiological situation below 5% or 3%. “And he adds:” You have to use the evidence with rationality. Of course, at the epidemiological moment that we have in Spain, it will not be an adequate strategy because, given the current incidence, it will be necessary to re-study everyone individually. “

This is also attested by a document from the Government of Chile, where group tests have also been used: “The convenience of applying ‘pool testing’ depends on the prevalence (the probability that a person has the virus). If the prevalence is low, it is advisable , since in this case, it will be more probable that group samples give negative and, therefore, it is not necessary to test individually “. On the other hand, “if the prevalence is high – greater than 30% – then it is not appropriate.”

Is it a new strategy?

The idea comes from World War II. This is when it was first used to detect syphilis. Later, in the 1980s, it worked for HIV. As the Ramn y Cajal Hospital microbiologist points out, “we have also used it to measure what is the circulation of the hepatitis virus, for example” and of course, with Covid, but in other contexts of lower incidence.

What other countries have used this massive test in Covid-19 times?

In Israel, Ghana, Singapore, China, Chile … In the United States, the FDA authorized a laboratory so that it could carry out emergency group tests and in Spain, it has worked with the ‘pool testing’ the Hospital of Vigo, for example . “It is something that was done in some moments when the incidence decreased. In fact, there are publications of Spanish groups that have validated it, but in other contexts with less incidence.”

Given that with the current incidence, it is not very cost-effective, perhaps later it can be more useful?

According to Canton Moreno, it is possible, but already associated with specific projects. That is, when the incidence drops and the cases are mostly asymptomatic (like one more respiratory virus), “it will not be of much interest as a method for screening or as a specific diagnosis, but it can be very interesting to carry out an epidemiological study in a population. concrete to dimension a concrete situation “.

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