One of the new concepts that came from Dr. Fauci’s testimony on Capitol Hill yesterday was the concept of “pool testing” to screen individuals for COVID-19 infection. This approach is a way to increase the number of people screened for viral infection or antibody without using additional tests. This approach can overcome shortages of tests by screening more than one person with an individual test.
Pool testing does not change the test technique or the reagents. You run the same test that you would do to screen for either virus (swab) or for antibody (blood) in a single individual. The difference is that instead of screening a single individual you combine a number of samples and screen the individuals together by mixing either their swabs or their blood samples. This saves time and reagents.
An example would be screening for viral infection in 100 people. If you screen each person individually, you have to run 100 tests on 100 swabs. If you screen ten groups of 10 people, you only have to run 10 tests. However, you then have to re-analyze each of the ten individuals in any of the positive samples. Unless all the pooled samples are positive, this approach uses fewer tests to screen the 100 individuals.
It should be obvious that there is value in pooled testing only when tests are in short supply and the incidence (frequency) of infection in the population is low. For example, if only 1% of individuals would be positive in the test, then by average is only one of the 10 pooled samples would it be positive; therefore, you would only have to repeat screen one pool or ten individuals (20 tests total).
In contrast, if the frequency of infection is 10% (above), then by chance every pool will be positive and you would have to screen all ten pools (100 individuals). In this example, you would actually use more tests (10+100) and also suffer from the time involved in running the samples twice.
I was somewhat surprised that this concept was raised in a congressional hearing about “hotspots” for COVID-19 infection. In the higher infection areas, the later example would predominate and pool testing would use more resources and take more time than individual testing. If one was combining 20 samples, as suggested in the hearing, then even with an incidence of infection of only 5% all of the pools would be positive!
In summary, pool testing is only of value in areas where the frequency of infection is very low and test kits are limited. That does not include any of the cities highlighted in tonight’s news!
Pandemic Pondering 