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.

Theres 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 Preventions coronavirus test, which is used by most public health laboratories in the United States, stipulates that pooling shouldnt be used when positivity rates exceed 10 percent. But at Mayo Clinic, wed have to start to question it once prevalence goes above 2 percent, definitely above 5 percent, Dr. Pritt said.

And prevalence isnt the only factor at play. The more individual samples grouped, the more efficient the process gets. But at some point, poolings 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.

Updated August 17, 2020

Are we going to cause harm if we miss them? I think thats 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 thats 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.

Originally posted here:

Why Pooled Testing for the Coronavirus Isn't Working - The New York Times