Split Pool Testing Ups Efficiency for SARS-CoV-2 Detection

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A “split pool” strategy for detecting SARS-CoV-2 in multiple samples could generate results faster than single test assays. The approach could also reduce the number of false positives and false negatives compared with currently approved pooled testing, new evidence suggests.

Dr Eugene Litvak

“It’s not too good to be true,” Eugene Litvak, PhD, lead author of an editorial outlining the new strategy, told Medscape Medical News. “This protocol requires far fewer tests and results in 10 times fewer false positives and false negatives” compared with single assays and FDA-approved pooled testing.

The editorial was published online September 24 in the American Journal of Public Health.

Pooled testing for syphilis as well as for HIV and other infectious agents has been used for decades to save time and resources. More recently, the FDA approved combined testing of up to five samples for SARS-CoV-2 for Quest and LabCorp under emergency use authorizations.

The Dorfman approach, approved in July, essentially allows labs to test combined samples using a single assay. If results come back positive for SARS-CoV-2, the individual samples are retested. If the pooled results are negative, each sample is considered to be free of the virus.

A drawback of the Dorfman approach is the potential to produce “high rates of false negative results,” Litvak and colleagues note. The danger, they add, is that people who are given a false negative finding might behave as if they were virus free.

Divide and Conquer?

The new approach, split pool testing, also combines samples for initial testing, but with modifications. All results are repeated; if a panel of 16 or 32 samples yields a negative overall finding, the same combined sample is tested again using the same or a different assay. If still negative, all tests in the sample are declared negative.

If a panel initially tests positive, the sample is split in half and retested for any remaining positives. This is done as many times as necessary to identify the presence of SARS-CoV-2, even down to a single test.

Litvak and colleagues evaluated the split testing strategy in a setting in which five million Americans per day are tested for the virus, as some experts recommend.

“We tested the protocol in a range of positivity rates, from 0.04% to 2.4%, so we are predicting two extremes,” said Litvak, who is president and CEO of the Institute for Healthcare Optimization in Newton, Massachusetts, and is affiliated with the Harvard T. H. Chan School of Public Health in Boston.

The average positivity rate across the United States is currently 1.2% to 1.4%, Litvak added.

Fewer False Negatives

The split pool approach yielded 60% of the false negative results compared with individual tests at both these low and higher prevalence rates.

Compared with individual sample testing, split pooling also would save resources — it requires only 10% of the number of tests at a 0.04% prevalence rate and only 41% of tests at a 2.4% prevalence level.

“When it comes to false negatives, the Dorfman protocol produces far worse results than split pooling — almost 10 times (9.8) as many as the split pooling method delivers in both prevalence situations,” the authors note.

In addition, split pool testing would reduce the number of false positive findings. Although split pool testing could still yield 6000 false positives at the lower prevalence rate, individual testing would generate 99,960 false positives, for example.

Split pool testing is “more efficient, much less expensive, and comparably more accurate,” Litvak said.

Litvak added a caveat. “When you get into a zone somewhere between a 10% to 20% prevalence rate, the test would not be useful. Almost every pooled test sample would be positive,” he added. “But it could definitely be used in the US now.”

“Given the rapidly rising number of infections now emerging in a number of states, it is surely time to try new strategies such as pooled testing. But by no means should pooled testing follow the Dorfman protocol,” the authors note.

They add, “We can’t afford to adopt strategies, such as individual testing and Dorfman pooling, that could run the risk of giving false assurances to large numbers of infected people, potentially making the toll of this terrible pandemic worse than it already is.”

More Value in Low-Prevalence Settings

“A pooled testing approach in low-risk populations and in low-prevalence areas can be valuable. However, false negative results in pooled sample testing, especially when the pools are relatively small, say four or five people, are unlikely to be important in areas where universal masking remains in effect,” Samir Shah, MD, told Medscape Medical News when asked to comment.

Dr Samir Shah

The split pooling protocol “seems to add the most value when large-scale testing is performed.

“I think scenarios of repeat or second round testing under the split pooling protocol will likely add a lot of cost but potentially only marginal benefit when large-scale testing of populations is not being done, as is the current situation in the US,” added Shah, professor of pediatrics and director of the Division of Hospital Medicine at Cincinnati Children’s Hospital Medical Center in Ohio.

Pooled testing in US schools and school athletics could be helpful in informing quarantine decisions to minimize the likelihood of student-related transmission, Shah said.

One concern remains the timely reporting of individual and pooled test results, he added. “If one has to wait several days for a test result, those results are less helpful,” he said.

Litvak and Shah have disclosed no relevant financial relationships.

Am J Public Health. Published online September 24, 2020. Editorial

Damian McNamara is a staff journalist based in Miami. He covers a wide range of medical specialties, including infectious diseases, gastroenterology and neurology. Follow Damian on Twitter: @MedReporter.

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