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Why Pooled Testing for the Coronavirus Isn't Working in the U.S.

Pooling accounts for about one-third of the samples that are processed at Poplar, Mr. Sweeney said, adding “that percentage is going to get much higher.”

But in many other regions, experts are having trouble clearing the hurdles to benefit from pooling — in part because needs differ so vastly from institution to institution, and even from test to test.

“There’s been a lot of concerns about all the challenges,” said Dr. Bobbi Pritt, director of the clinical parasitology laboratory at Mayo Clinic, which processes tens of thousands of coronavirus tests each week, but has yet to roll out pooling.

Experts disagree, for instance, on the cutoff at which pooling stops being useful. The Centers for Disease Control and Prevention’s coronavirus test, which is used by most public health laboratories in the United States, stipulates that pooling shouldn’t be used when positivity rates exceed 10 percent. But at Mayo Clinic, “we’d have to start to question it once prevalence goes above 2 percent, definitely above 5 percent,” Dr. Pritt said.

And prevalence isn’t the only factor at play. The more individual samples grouped, the more efficient the process gets. But at some point, pooling’s perks hit an inflection point: A positive specimen can only get diluted so much before the coronavirus becomes undetectable. That means pooling will miss some people who harbor very low amounts of the virus.

“Are we going to cause harm if we miss them? I think that’s still a difficult question to answer,” Dr. Liesman said. These people may be less likely to pass the virus to others, and may be at lower risk of getting severely ill. But that’s no guarantee. Some might simply be early on in their infection.

Pooling can also be onerous for lab technicians — many of whom have been working grueling hours for months on end. Though simple in theory, batching samples is tedious and time-consuming, as researchers carefully transfer precise amounts of liquid from one tube to another hundreds, perhaps thousands, of times over.

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