In the last two weeks of March and early April 2, when the first peak of cases was recorded COVID-19 in the United States, it was sometimes difficult to access a test for SARS-CoV-2: there were not enough kits, or reagents were missing, or swabs were missing, or there was no physical space to take the samples or the laboratories were unable to process them, or several of those problems at the same time. To avoid that in July, when the country faces another critical rise in cases (total infections already exceed 4.0 million, with 144,958 deaths accumulated), something similar happens, the Food and Drug Administration ( FDA ) authorized a form of testing that allows reaching more people in less time and with less cost.
On Saturday, July 18, with an emergency approval, the FDA allowed the Quest Diagnostics laboratory to begin using a “sample pool” process: the combination of a group of multiple-person swabs for testing, rather than one, by several. This speeds up the testing process: Wuhan, the original epicenter of the coronavirus, applied this technique to reach almost all of its inhabitants.
When a group of several people tests negative, it can be established that all of them are free of SARS-CoV-2; when positive, it is necessary to test each sample individually to find out who is infected. “It is a really good tool,” said Anthony Fauci, head of the National Institute of Allergy and Infectious Diseases ( NIAID ). “It can be used in various circumstances, including community level or schools .”
And in June, Deborah Birx, coordinator of the Special Group on Response to the Coronavirus of the White House, said in a virtual conference of the National Association of Microbiology of the United States: “The pool of samples would give us the capacity to exceed half a million of tests per day to potentially 5 million people analyzed each day “.
For example, when reopening economic activity, it allows faster analysis in the workplace. On the other hand, it is not useful in foci such as nursing homes, where the probability that there is always a positive in the collective sample increases. It is generally advised when less than 10% of people tested are expected to be positive.
It is, as Stat, a science and medicine publication explained, “part of a comprehensive disease surveillance strategy,” not an individual diagnosis. “It allows the constant detection of people who do not show symptoms of COVID-19. The analysis of asymptomatic people is important because a large part of people who have coronavirus do not show symptoms or it takes a few days to start feeling sick, but they can still spread it ”.
For example, in a warehouse with 100 employees, the company could regularly test COVID-19 on staff, and instead of doing 100 tests each time, it could send the samples in sets of 10 and do only 10: that would allow reducing costs and time, and would facilitate greater regularity in testing, at least from the point of view of resources.
“If one of those groups tested positive, you could do an individual test on those 10 employees to find out who was infected,” Stat said. While waiting for the result, 10 should be separated from work. Instead, “the other 90 employees, in the negative groups, would not need to be examined again. ” The objective would be “to try to detect a case before the person can spread the coronavirus to others”, something of enormous utility in the fight against the pandemic: “Scientists discover that more and more this type of large-transmission super-transmission event scale, in workplaces, restaurants, and bars or places of worship, drive much of the transmission.
Birx added in his lecture that the sample pool method enables much more active contact tracing by finding asymptomatic or pre-symptomatic infections early. “It allows for more frequent testing in a population that may have a low incidence of the disease,” Benjamin Pinsky, medical director of the Stanford University Clinical Virology Laboratory, who has led cluster studies for the coronavirus, told Stat. “That would allow us to test many negatives, but also identify those infected before they develop symptoms.”
The technology used in pooled samples is the same as that used in individual diagnoses, which identifies genetic traits of SARS-CoV-2, the reverse transcriptase-polymerase chain reaction ( RT-PCR ). The number of people that can be grouped in a sample pool depends on the capacity of the machines in which the tests are processed, but in some cases, it can reach 10 samples.
This methodology has a downside, however: it increases the risk of false negatives. “If someone has the virus, the viral load in their particular sample will be diluted by combining with the others, perhaps to the point where the machine can no longer detect it,” Stat illustrated. “One way to offset the risk of false negatives is to test frequently, experts say.”
This regularity facilitated by the sample pool could also function as an indicator of the increase in the spread: if suddenly a higher percentage of the groups begin to test positive, it could be considered as a signal for the authorities to increase social distance strategies before hospitalizations go up.
Once there is an identified outbreak, however, the sample pool loses utility. Today, for example, it might make sense in states like Massachusetts or New York, but not so much anymore in Florida or Texas.