Pooled testing for COVID-19 presents promising pitfalls

Pooled testing for COVID-19 presents promising pitfalls


The country’s top health officials are counting on a new approach to dramatically improve U.S. screening for the coronavirus: combining test samples in batches rather than running them one at a time.

The potential benefits include stretching lab supplies, cutting costs, and extending testing to millions more Americans who unwittingly spread the virus. Health officials believe that infected people who show no symptoms are largely responsible for the rising number of cases in more than half of the states.

“Pooling would give us the ability to go from half a million tests per day to potentially 5 million individuals per day,” said Dr. Deborah Birx, the White House coronavirus response coordinator, at a recent meeting of laboratory experts.

For the time being, federal health regulators have not released any laboratories or test manufacturers to use the technique. The Food and Drug Administration released guidelines for test makers in mid-June and wants each to first show that mixing samples doesn’t compromise accuracy, one of the potential drawbacks.

So it’s not clear when pooled tests may be available for mass showings in schools and businesses.

The principle is simple: instead of running each person’s tests individually, labs would combine parts of different people’s nasal samples and test them together. A negative result would remove everyone in the batch. Each sample must be retested individually for a positive result. Pooling works best with lab tests, which take hours – not the much faster individual tests used in clinics or doctor’s offices.

The idea of ​​pooling dates back to World War II, when it was considered to quickly screen blood samples from U.S. conscripts for syphilis. It has since been used to screen blood samples for HIV and hepatitis. And developing countries have used pooled samples to stretch test supplies.

China reported using the approach as part of a recent campaign to test all 11 million residents of Wuhan, the city where the virus first appeared at the end of last year.

“Americans think this is a new concept because we don’t normally have the challenge of expanding testing capacity,” said Darius Lakdawalla, health economist at the University of Southern California.

Lakdawalla and colleagues estimate that pooled tests can save schools and businesses between 50% and 70% costs. Under their model, a group of 100 workers could be divided into 20 batches of five. Assuming 5% of people carry the virus, only five pools would test positive, requiring individual testing. Ultimately, 45 tests would be required for the pooled approach, compared to 100 individual tests.

But pooling is not always the best option. Importantly, it doesn’t save time or resources when used in COVID-19 hotspots, such as a nursing home outbreak. This is because the logistical and financial benefits of pooling only increase if a small number of pools are positive.

Experts recommend the technique when less than 10% of people are expected to test positive. About 7% of U.S. tests were positive for the virus in the past week, according to an AP analysis, although rates vary widely from place to place. For example, pooling would not be cost effective in Arizona, where an increase has pushed positive test results to over 22%. But the approach could make sense in New Jersey, with a positivity rate of less than 2%.

The Nebraska health lab used batch tests in March with special permission from the governor and the FDA. The lab director said they had to quit a few weeks ago when their positive rate rose to 17% due to outbreaks at meat processing plants.

“We knew that pooling no longer worked when those rates started to rise,” said Dr. Peter Iwen.

Reserving pooled tests for large groups with low infection rates is in line with increasing government attention to people without symptoms spreading the virus, especially young people.

“It is a very good tool. It can be used in a number of circumstances, including at community level or even in schools, ‘said Dr. Anthony Fauci, the country’s top infectious disease expert, Tuesday at a Senate hearing.

However, health officials may still need to convince some key players to apply the method. LabCorp, one of the largest test chains in the country, said in an email that it is familiar with pooled tests, but currently believes that “testing individual patients is the most effective and efficient way” to test for COVID-19.

Dr. Colleen Kraft of Emory University is concerned that batch testing – with its multiple rounds of screening for some patients – could delay test results, a key factor in quarantining infected people.

“If you try to do something quickly, it will extend the turnaround time,” Kraft said.

She and others are also concerned about accuracy, as test performance tends to decrease when screening in larger groups of people where the target disease is less common.

“If we can’t trust the test results, there’s no point in taking the test,” said Jennifer Nuzzo, of Johns Hopkins University’s Covid-19 Testing Insights Initiative.


Follow Matthew Perrone on Twitter: @AP_FDAwriter


AP writers Lauran Neergaard in Washington, Nicky Forster in New York and Christina A. Cassidy in Atlanta contributed to this report.


The Associated Press Health and Science Department is supported by the Department of Science Education of the Howard Hughes Medical Institute. The AP is solely responsible for all content.

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