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Aug. 21, 2020, 6:29 p.m. EDT

Why pooled testing could be a breakthrough in the fight against COVID-19

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By Darius Lakdawalla and Erin Trish

Hopes for a summertime reprieve from COVID-19 have been dashed as cases surged in June. As infections rise, so does the need for testing.

In late June, Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, announced that health officials were considering  pooled testing  for COVID-19 in response to the recent surge. On July 18, the Food and Drug Administration issued an emergency use authorization to Quest Diagnostics /zigman2/quotes/201001842/composite DGX +0.50% that would allow the lab testing company to  pool samples from up to four patients  to test for the coronavirus.

So what is pooled testing, and why does the U.S. need it?

The basic idea of  pooled testing  is that it allows public health officials to test small groups—called pools—of people using only one test. This means you can test more people faster, using fewer tests and for less money.

Instead of testing one person at a time, samples from multiple individuals would be mixed together and tested as one. If the test comes back negative, everyone in the pool is clear. If positive, each member of the pool is then tested individually.

For example, imagine a workplace with 20 people, one of whom is infected. Pooled testing divides the 20 people into four groups of five. Samples are taken from each person and mixed with the samples from other people in their group. Each grouped sample is then tested—four tests in all.

One of the pools will turn up positive because it contains the single infected person. All five people in this pool are then retested individually, and the one sick person is identified.

In all, nine tests are used and all 20 workers have been screened. If everyone had been screened individually, it would have taken 20 tests.

In practice, the number of tests saved by pooling varies  depends on the fraction of people infected . Higher infection rates mean that more pools come back positive, more people need to be retested, and savings from pooling are lower.

For example, pooled testing on suspected COVID-19 cases in Los Angeles—where as of June 27  nearly 9% of people tested are positive —would reduce test use by about 50%. But in Montana, where only  0.1% of the population is infected , a pooled testing effort to test the whole state would reduce test use by  nearly 90% .

No matter the exact amount, at  a cost of about $100 per test , the savings add up.

Pooled testing has long been used to test large asymptomatic populations for disease. State labs have used it  to screen for chlamydia and gonorrhea , and the Red Cross has used it  to test donated blood  for Hepatitis B and C, Zika virus and HIV.

And some places have already begun using pooled testing to screen for active coronavirus infections.

Stanford University  used pooled testing  to monitor the early spread of COVID-19 in the Bay Area. Nebraska’s Public Health Laboratory  pooled and tested samples from five-person groups  and  roughly doubled  the number of suspected COVID-19 patients it could test weekly.

Some people have expressed concern that the dilution of samples from pooling could reduce the accuracy of the PCR tests that look for viral RNA in samples. Fortunately, researchers have already shown that pooled testing is about as accurate as individual testing  in pools as large as eight people .

Another concern is infection prevalence. When infection rates  climb higher than 15% , too many pools come back positive and pooling is no longer more efficient. Fortunately, on a nationwide basis only about  5% of tests  on suspected cases are coming back positive. Even in extreme hot spots like Texas, where  10% or more of suspected cases are testing positive , pooled testing could still be useful.

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