The Pros and Cons of Pooling Covid-19 Tests

(Bloomberg) -- Slowing the spread of the novel coronavirus requires testing -- lots of it. The need is particularly acute for this virus because many may be asymptomatic and unknowingly spreading it to others. Though millions of people around the world have been tested, supplies aren’t keeping up with demand. One innovative approach, called pool testing, would multiply the reach of existing capacity. Few countries have adopted the idea so far.

1. How does pool testing work?

Testing for Covid-19 -- the disease caused by the coronavirus -- is done with samples from saliva or swabs of a person’s nose or throat. In pool testing, samples are grouped into one batch and subjected to a single test. If it comes up negative, no further action is taken, sparing individual testing. only if the pool comes up positive is each person individually tested to determine who has the virus. Pool testing is especially helpful for detecting those who are infected but have no symptoms of Covid-19.

2. Who is pool testing for?

Communities with relatively low incidence of coronavirus would benefit the most. Hot spots such as Arizona, where nearly a quarter of tests come back positive, or Brazil, one of the epicenters of the outbreak, wouldn’t benefit because many if not all of the pools would come back positive, forcing everyone to be tested individually anyway.

3. Does it work?

Pooling increases the capacity and speed of testing efforts and reduces costs. But its efficacy depends on how many samples are combined at one time. The more samples the batch contains, the more diluted any positive ones are, increasing the chance of a false negative. Of course, false negatives also occur with individual testing.

4. Where has pool testing been tried?

The Chinese city of Wuhan, where the coronavirus first emerged late in 2019, went from conducting 46,000 tests a day to nearly 1.47 million by grouping samples. It was able to test 6.5 million people in two weeks in May. In Germany, pools ranging in size from four to 30 samples were used to test nursing home residents and staff. India tests migrant works and international passengers in pools of 25. Using pool testing, Ghana has one of the highest testing rates in Africa at about five per 1,000. In the U.S., early in the epidemic, health authorities in Nebraska used the pool method when they ran low on testing ingredients, and Stanford University researchers employed it to track the spread in their region. However, the U.S. Food and Drug Administration has not yet approved the method for wide use.

5. Has it been deployed in other outbreaks?

In the U.S., pool testing was used during World War II to test members of the Army for syphilis. Pool testing was also used to test for HIV during the epidemic that hit the U.S. in the mid-1980s when tests were limited and expensive.

The Reference Shelf

• QuickTake explainers on asymptomatic “silent spreaders” of the virus, unanswered questions about this coronavirus and the next pandemics.
• A professor of mathematics made the case for pool testing in a New York Times op-ed.
• One study found 13 to be the optimal number of samples to be pooled for Covid-19 testing.

©2020 Bloomberg L.P.